agent: Allow threads to interrupt main select loop with SIGCONT.

* agent/gpg-agent.c (interrupt_main_thread_loop): New function on
non-windows platforms, allows other threads to interrupt the main loop
if there's something that the main loop might be interested in.

For example, the main loop might be interested in changes in program
state that affect the timers it expects to see.

I don't know how to do this on Windows platforms, but i welcome any
proposed improvements.

Gbp-Pq: Topic gpg-agent-idling
Gbp-Pq: Name agent-Allow-threads-to-interrupt-main-select-loop-wi.patch

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