fd-util: Expose helper to pack fds into 3,4,5,...

This is useful for situations where an array of FDs is to be passed into
a child process (i.e. by passing it through safe_fork). This function
can be called in the child (before calling exec) to pack the FDs to all
be next to each-other starting from SD_LISTEN_FDS_START (i.e. 3)

src/shared/exec-util.c

@@ -539,7 +539,7 @@ int fork_agent(const char *name, const int except[], size_t n_except, pid_t *ret
 
         r = safe_fork_full(name,
                            NULL,
-                           except,
+                           (int*) except, /* safe_fork_full only changes except if you pass in FORK_PACK_FDS, which we don't */
                            n_except,
                            FORK_RESET_SIGNALS|FORK_DEATHSIG_SIGTERM|FORK_CLOSE_ALL_FDS|FORK_REOPEN_LOG|FORK_RLIMIT_NOFILE_SAFE,
                            ret_pid);