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.c

@@ -2,11 +2,42 @@
 
 #include "format-table.h"
 #include "parse-argument.h"
+#include "path-util.h"
 #include "signal-util.h"
 #include "stdio-util.h"
 #include "string-table.h"
 #include "string-util.h"
 
+/* All functions in this file emit warnigs. */
+
+int parse_path_argument(const char *path, bool suppress_root, char **arg) {
+        char *p;
+        int r;
+
+        /*
+         * This function is intended to be used in command line parsers, to handle paths that are passed
+         * in. It makes the path absolute, and reduces it to NULL if omitted or root (the latter optionally).
+         *
+         * NOTE THAT THIS WILL FREE THE PREVIOUS ARGUMENT POINTER ON SUCCESS!
+         * Hence, do not pass in uninitialized pointers.
+         */
+
+        if (isempty(path)) {
+                *arg = mfree(*arg);
+                return 0;
+        }
+
+        r = path_make_absolute_cwd(path, &p);
+        if (r < 0)
+                return log_error_errno(r, "Failed to parse path \"%s\" and make it absolute: %m", path);
+
+        path_simplify(p, false);
+        if (suppress_root && empty_or_root(p))
+                p = mfree(p);
+
+        return free_and_replace(*arg, p);
+}
+
 int parse_signal_argument(const char *s, int *ret) {
         int r;