nsc: Replace NSC_EXTRA, NSC_CONST by enum ca_dump

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>