lib: make prefix2str simpler to use, and use it in zclient

lib: make prefix2str simpler to use, and use it in zclient

Returning the buffer allows using it in the logging functions
in easier way. This also makes the API consistent with sockunion.

Add also PREFIX_STRLEN to be the generic buffer length required
for any prefix string representation.

Signed-off-by: Timo Teräs <timo.teras@iki.fi>
Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from commit 41eb9a4305fbcb206c900a18af7df7115d857d60)