Signal handling: switch servers, reload config, etc.

A useful but missing feature: when the currently connected server is slow, the user should be able to tell sslocal to switch to another server immediately.

We can take a leaf out of tor's signal handling book:

SIGHUP
    The signal instructs Tor to reload its configuration (including closing and
    reopening logs), and kill and restart its helper processes if applicable.

SIGUSR1
    Log statistics about current connections, past connections, and throughput.

SIGUSR2
    Switch all logs to loglevel debug. You can go back to the old loglevels by
    sending a SIGHUP.

Analogously, sslocal would remember the config passed via -c, and reload it upon receiving SIGHUP. We can use SIGUSR1 for "switch servers now" (perhaps lowering the "score" of the previous server, too, basically marking it as kind of slow right now... if sslocal keeps such a "score" on servers, that is. And if so, SIGHUP should also reset all server scores.)

What do you think?

Start by locating sslocal's signal-handling and configuration entry points, especially how the config passed via -c is retained. Define the reload and server-switch behavior for SIGHUP and SIGUSR1, including any server scoring and logging implications. Done means the behavior is implemented and covered by tests, though the issue names no existing files or tests.