glob-util: rework safe_glob()

glob-util: rework safe_glob()

Currently, callers of safe_glob() set an empty glob_t or glob_t with
opendir func, and all other components are always zero.
So, let's introduce safe_glob_full() which optionally takes opendir
function, rather than glob_t, and returns result strv, rather than
storing results in glob_t.
Also, introduce safe_glob() which is a trivial wrapper of
safe_glob_full() without opendir func.

No functional change, just refactoring.