libsepol: fix most gcc -Wwrite-strings warnings

gcc puts literal strings lie in read-only memory.  On x86_64, trying to
write to them triggers a segmentation fault.

To detect such issues at build time, variables holding a pointer to such
strings should be "const char*".  "gcc -Wwrite-strings" warns when using
non-const pointers to literal strings.

Remove gcc warnings by adding const to local variables and argumens of
internal functions.

This does *not* fix this warning:

  policydb_public.c:208:10: warning: passing argument 2 of 'hashtab_search' discards 'const' qualifier from pointer target type
    return (hashtab_search(p->p.p_classes.table, PACKET_CLASS_NAME) ==
            ^
  In file included from ../include/sepol/policydb/symtab.h:16:0,
                   from ../include/sepol/policydb/policydb.h:60,
                   from policydb_public.c:4:
  ../include/sepol/policydb/hashtab.h:98:24: note: expected 'hashtab_key_t' but argument is of type 'const char *'
  extern hashtab_datum_t hashtab_search(hashtab_t h, const hashtab_key_t k);
                         ^

Moreover the "const" word in hashtab_search prototype does not make the
second parameter "const char*" but "char* const".

Acked-by: Steve Lawrence <slawrence@tresys.com>
6 files changed