allow the attach name to be specified in srv(4).
this is helpful when writing rules for 9fs that need
an attachpoint.  the standard way of doing this

case aggle
	srv $nflag -q tcp!aggle.example.org aggle &&
		mount $nflag /srv/aggle /n/aggle aggle

suffers the problem that if /srv/aggle becomes stale, it is
not retried.  with this change this rule can be rewritten
like so

case aggle
	srv $nflag -q tcp!aggle.example.org aggle /n/aggle aggle

and it is now correct, even if /srv/aggle becomes stale.