multipath: don't start multipathd unnecessarily

multipath: don't start multipathd unnecessarily

When multipathd is off, but systemd is monitoring the multipathd socket,
mpath_connect() has the side effect of starting multipathd. That's
superfluous if we aren't going to pass the command at hand to multipathd.
So, only try to open the socket if the command at hand is a candidate
for delegation.

Note: Despite this, some day in the future, we may decide to delegate
almost everything to multipathd.

Fixes: 506d253b "multipath: delegate dangerous commands to multipathd"
Signed-off-by: Martin Wilck <mwilck@suse.com>