Continued refactoring getters into setters.

Prominent getters refactored in this tilting at windmills include
convert_string_to_glob_capturing_substrings (to :enable_glob_capturing()) and
:prefix_string(). Arguably more critically, is_string_matches_glob() has been
significantly optimized to avoid both unnecessary subshells and coprocesses,
thereby also eliminating the prior is_string_matches_glob_safely() helper.