Move and rename parse_path_argument() function

This fits better in shared/, and the new parse-argument.c file is a good home
for it.

src/shared/parse-argument.h

@@ -1,4 +1,5 @@
 /* SPDX-License-Identifier: LGPL-2.1-or-later */
 #pragma once
 
+int parse_path_argument(const char *path, bool suppress_root, char **arg);
 int parse_signal_argument(const char *s, int *ret);