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.

src/basic/glob-util.h

@@ -5,11 +5,15 @@
 
 #include "forward.h"
 
-/* Note: this function modifies pglob to set various functions. */
-int safe_glob(const char *path, int flags, glob_t *pglob);
+typedef DIR* (*opendir_t)(const char *);
+
+int safe_glob_full(const char *path, int flags, opendir_t opendir_func, char ***ret);
+static inline int safe_glob(const char *path, int flags, char ***ret) {
+        return safe_glob_full(path, flags, NULL, ret);
+}
 
 /* Note: which match is returned depends on the implementation/system and not guaranteed to be stable */
-int glob_first(const char *path, char **ret_first);
+int glob_first(const char *path, char **ret);
 #define glob_exists(path) glob_first(path, NULL)
 int glob_extend(char ***strv, const char *path, int flags);