Make sector types signed

get_empobj_chr() and emp_obj_chr_name() access struct sctstr member
sct_type through struct empobj member type.  This is technically
non-portable, because the two differ in signedness.  It was also
undocumented.  Fix by making sct_type signed.  sct_newtype as well,
for consistency.

map_char() uses unsigned char for a sector type argument.  Change that
to int.  Matches how this is done elsewhere.

include/empobj.h

@@ -56,7 +56,8 @@ struct empobj {
     short uid;
     coord x;		/* is valid if EFF_XY      is set in table def. */
     coord y;		/* is valid if EFF_XY      is set in table def. */
-    signed char type;
+    signed char type;	/* is valid for sectors and units */
+    /* remaining are valid for units */
     signed char effic;
     signed char mobil;
     unsigned char off;