core: Only scan for the command line if probably on the same host.

* pinentry/pinentry.c (pinentry_get_title): Check the current hostname
and make sure it matches.  If it does not, do not bother looking for
the command line.

If we don't do this, and the agent is forwarded from somewhere else,
pinentry will be looking up arbitrary process command lines.

• Signed-off-by: Daniel Kahn Gillmor <dkg@fifthhorseman.net>