Refactor: Use 'const' in a few more places.

Refactor: Use 'const' in a few more places.

* gnulib-local/lib/mem-hash-map.h (hash_find_entry): Mark htab as a 'const' pointer.
* gnulib-local/lib/mem-hash-map.c (lookup, hash_find_entry): Likewise.
* gettext-tools/src/message.h (message_list_search): Mark mlp as a 'const' pointer.
* gettext-tools/src/message.c (message_list_search): Likewise.
* gettext-tools/src/msgl-ascii.h (is_ascii_string_list): Mark slp as a 'const' pointer.
(is_ascii_message): Mark mp as a 'const' pointer.
(is_ascii_message_list): Mark mlp as a 'const' pointer.
(is_ascii_msgdomain_list): Mark mdlp as a 'const' pointer.
* gettext-tools/src/msgl-ascii.c (is_ascii_string_list): Mark slp as a 'const' pointer.
(is_ascii_message): Mark mp as a 'const' pointer.
(is_ascii_message_list): Mark mlp as a 'const' pointer.
(is_ascii_msgdomain_list): Mark mdlp as a 'const' pointer.
* gettext-tools/src/msgl-ofn.h (message_has_filenames_with_spaces): Mark mp as a 'const' pointer.
(message_list_has_filenames_with_spaces): Mark mlp as a 'const' pointer.
(msgdomain_list_has_filenames_with_spaces): Mark mdlp as a 'const' pointer.
* gettext-tools/src/msgl-ofn.c (message_has_filenames_with_spaces): Mark mp as a 'const' pointer.
(message_list_has_filenames_with_spaces): Mark mlp as a 'const' pointer.
(msgdomain_list_has_filenames_with_spaces): Mark mdlp as a 'const' pointer.
* gettext-tools/src/msgl-check.h (check_message_list): Mark mlp as a 'const' pointer.
* gettext-tools/src/msgl-check.c (check_plural, check_message_list): Likewise.