server/admin.c, server/tripe-admin.5.in: Fix PEERINFO segfault.

It's allowed for p_privtag to return null, so just trying to print it is
a bad idea.  This introduces a protocol change, since there is no single
tag chosen in the absence of `-priv' or `-t' options -- instead, there's
a search list; this is represented with the special tag `(default)'.