Put proper logging into Pageant.

Put proper logging into Pageant.

Now it actually logs all its requests and responses, the fingerprints
of keys mentioned in all messages, and so on.

I've also added the -v option, which causes Pageant in any mode to
direct that logging information to standard error. In --debug mode,
however, the logging output goes to standard output instead (because
when debugging, that information changes from a side effect to the
thing you actually wanted in the first place :-).

An internal tweak: the logging functions now take a va_list rather
than an actual variadic argument list, so that I can pass it through
several functions.