nsc: Replace NSC_EXTRA, NSC_CONST by enum ca_dump

struct castr ca_flag NSC_EXTRA was introduced in commit 3e5c064
(v4.2.18) to permit selectors that aren't in xdump.

Flag NSC_CONST was introduced in commit 445dfec, and put to use in
commit d8422ca (both v4.3.0) to protect certain table elements that
should not be changed in customized tables.

Both flags apply only to xdump, not to other uses of struct castr,
such as conditionals.

Combining NSC_EXTRA | NSC_CONST makes no sense.

I'll shortly need a way to keep selectors out of configuration tables
for conditional selector and xdump command forward compatibility.
Doing it as a third flag would add more nonsensical combinations.

Convert the flags to a separate enum ca_dump instead:

    neither   -> CA_DUMP
    NSC_CONST -> CA_DUMP_CONST
    NSC_EXTRA -> CA_DUMP_NONE

Bonus: unlike the flags it replaces, ca_dump is not visible in xdump.

Signed-off-by: Markus Armbruster <armbru@pond.sub.org>

doc/xdump

@@ -256,8 +256,6 @@ of xdump meta T are:
 
 * flags: The field's flags, a symbol set.  Flags are:
   - "deity", field visible only to deities
-  - "extra", field not to be dumped
-  - "const", field cannot be changed
   - "bits", field is a symbol set, field type must encode symbol "d",
     field table must not be -1.
   - "hidden", field value is masked for contact when option HIDDEN is
@@ -298,8 +296,8 @@ in other places as well:
   for another table (new struct castr member ca_table).  The selector
   code doesn't use that, yet.
 
-* Selector flag NSC_EXTRA to flag redundant selectors, so that xdump
-  can ignore them.
+* Redundant selectors can be marked so that xdump ignores them (new
+  struct castr member ca_dump).
 
 Meta-data is in empfile[] (table meta-data), src/lib/global/nsc.c
 (selector meta-data), src/lib/global/symbol.c (symbol tables).  The
@@ -313,12 +311,12 @@ its meta-data, and to make sense of that table, we need meta-meta
 data.  So we start with that:
 
     [3:640] Command : xdump meta meta
-    XDUMP meta meta 1464537892
-    "name" 3 4 0 -1
-    "type" 8 4 0 33
-    "flags" 8 12 0 32
-    "len" 7 4 0 -1
-    "table" 8 4 0 26
+    XDUMP meta meta 1464554085
+    "name" 3 0 0 -1
+    "type" 8 0 0 33
+    "flags" 8 8 0 32
+    "len" 7 0 0 -1
+    "table" 8 0 0 26
     /5
 
 To interpret this table, we have to know the field names and their
@@ -342,9 +340,9 @@ know, namely the table of tables.  Let's dump that next, starting with
 its meta-data:
 
     [3:640] Command : xdump meta table
-    XDUMP meta table 1464537892
+    XDUMP meta table 1464554085
     "uid" 8 0 0 26
-    "name" 3 4 0 -1
+    "name" 3 0 0 -1
     /2
 
 Because xdump table is referenced from elsewhere (xdump meta meta
@@ -354,7 +352,7 @@ table itself.  Indeed, its value matches the one we got in xdump meta
 meta.  Let's try to dump the table:
 
     [5:640] Command : xdump 26 *
-    XDUMP table 1464537892
+    XDUMP table 1464554085
     0 "sect"
     1 "ship"
 [...]
@@ -373,13 +371,14 @@ Now dump the two symbol tables we postponed.  Because xdump accepts
 table IDs as well as names, we don't have to know their names:
 
     [5:640] Command : xdump meta 33
-    XDUMP meta meta-type 1464537892
-    "value" 8 4 0 -1
-    "name" 3 4 0 -1
+    xdump meta 33
+    XDUMP meta meta-type 1464554085
+    "value" 8 0 0 -1
+    "name" 3 0 0 -1
     /2
 
     [6:640] Command : xdump 33 *
-    XDUMP meta-type 1464537892
+    XDUMP meta-type 1464554085
     1 "d"
     2 "g"
     3 "s"
@@ -396,19 +395,17 @@ table IDs as well as names, we don't have to know their names:
     /13
 
     [7:640] Command : xdump meta 32
-    XDUMP meta meta-flags 1464537892
-    "value" 8 4 0 -1
-    "name" 3 4 0 -1
+    XDUMP meta meta-flags 1464554085
+    "value" 8 0 0 -1
+    "name" 3 0 0 -1
     /2
 
     [7:640] Command : xdump 32 *
-    XDUMP meta-flags 1464537892
+    XDUMP meta-flags 1464554085
     1 "deity"
-    2 "extra"
-    4 "const"
     8 "bits"
     16 "hidden"
-    /5
+    /3
 
 We now have complete meta-meta information:
 
@@ -424,10 +421,10 @@ Dumping the remaining tables is easy: just walk the table of tables.
 Here's the first one:
 
     [7:640] Command : xdump meta 0
-    XDUMP meta sect 1464537892
+    XDUMP meta sect 1464554085
     "owner" 5 0 0 8
-    "xloc" 9 4 0 -1
-    "yloc" 10 4 0 -1
+    "xloc" 9 0 0 -1
+    "yloc" 10 0 0 -1
     "des" 4 0 0 18
 [...]
     /78
@@ -621,8 +618,9 @@ symbol table packing.
 
 Configuration tables contain values that are not meant to be
 customized.  For instance, meta-data and symbol tables reflect the
-encoding of C language constructs in the server.  Selector flag
-NSC_CONST marks them, so that the code can prohibit changes.
+encoding of C language constructs in the server.  Such selectors are
+marked (struct castr member ca_dump), so that the code can prohibit
+changes.
 
 All tables are checked against meta-data on server startup by
 ef_verify().  More elaborate checking would be nice, and probably