Revisit the locating-rule code.

Revisit the locating-rule code.

* gettext-tools/src/search-path.c: Reorder definitions. Add comments.
(get_search_path): Use XNMALLOC instead of XCALLOC. Verify the memory
allocation.
* gettext-tools/src/locating-rule.h: Add comments.
(locating_rule_list_locate): Add const to first argument.
* gettext-tools/src/locating-rule.c: Add comments.
(locating_rule_list_locate): Add const to first argument.
* gettext-tools/src/FILES: Update.
* gettext-tools/doc/gettext.texi (Preparing ITS Rules): Fix a typo.