intl: Sync up loadmsgcat.c with glibc
* loadmsgcat.c: Include assert.h.
(_nl_load_domain): Add semicolon to end of line.
(_nl_load_domain)[HAVE_MMAP]: Use MAP_FAILED and assert that
it is (void *) -1.
(_nl_load_domain)[_LIBC]: Call __libc_rwlock_init instead of
gl_rwlock_init.
(_nl_unload_domain): Cast convd->encoding to non-const.
