fd-util: add new FORMAT_PROC_PID_FD_PATH() helper

This is just like FORMAT_PROC_FD_PATH() but goes via the PID number
rather than the "self" symlink.

This is useful whenever we want to generate a path that is useful
outside of our local scope.

src/basic/fd-util.c

@@ -988,3 +988,11 @@ const char *accmode_to_string(int flags) {
                 return NULL;
         }
 }
+
+char *format_proc_pid_fd_path(char buf[static PROC_PID_FD_PATH_MAX], pid_t pid, int fd) {
+        assert(buf);
+        assert(fd >= 0);
+        assert(pid >= 0);
+        assert_se(snprintf_ok(buf, PROC_PID_FD_PATH_MAX, "/proc/" PID_FMT "/fd/%i", pid == 0 ? getpid_cached() : pid, fd));
+        return buf;
+}