zebra: fix optional distance on static mrib route

zebra: fix optional distance on static mrib route

Unfortunately, the quagga CLI parser doesn't support [<1-255>].  Fix by
working around with an alias.

Replaces the following commits:
- zebra: mrib: [no] ip mroute - require distance.
- zebra: mrib: [no] ip mroute - make distance optional.
(Rewritten as alias)

Cc: Everton Marques <everton.marques@gmail.com>
Cc: Balaji G <balajig81@gmail.com>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>