Document -t as not supported by Pageant

... rather than calling it not implemented.  This wording was already in
the manpage, but now it's in --help and the usage warning too.
Add -r/--reuse to test for an existing socket

This probes with connect() to see if the given socket is listening, and
if so just assumes that an agent is already alive.  If not, it tries to
start itself as normal.

Closes GH-18.