Clean up GCC warnings.

* src/reader.c (copy_action): `buf' is not used.
(parse_skel_decl): Be static.
* src/muscle_tab.c (mhash1, mhash2, muscle_insert): Preserve `const'.
* src/options.h (create_long_option_table): Have a real prototype.
* lib/hash.c, lib/hash.h (hash_insert, hash_insert_at, hash_delete)
(hash_delete_at): Return const void *.
Adjust casts to preserve the const.