search-path.c: Improve code style.
* gettext-tools/src/search-path.h (get_search_path): Clarify memory allocation.
* gettext-tools/src/search-path.c (struct path_array_ty): Add comment.
(foreach_elements): Fix comment.
(get_search_path): Improve comments. Perform each getenv() only once. Reduce
scope of variables. Fix gcc warning.
