Regularly check for user input when searching, to have a chance to catch EOF from...

Regularly check for user input when searching, to have a chance to catch EOF from engine.

When an engine quits without terminating a searching gnushogi, it would
continue to search and never realize there was no reason to continue.

This is a poll-based implementation of what H.G.Muller wrote for WIN32 to
circumvent the lack of UNIX signals there.