fd-util: when re-opening a directory with fd_reopen() go via openat(…, ".", …)

fd-util: when re-opening a directory with fd_reopen() go via openat(…, ".", …)

This adds a tiny shortcut to fd_reopen(): if we are about to reopen the
fd via O_DIRECTORY then we know it#s a directory and we might as well
reopen it via opening "." using the fd as "at fd" in openat().

This has the benefit that we don't need /proc/self/fd/ around for this
special case: fewer sources of errors.