Patch by Masayuki Yamamoto, reviewed and rebased by Erik Bray.

This is a first step on the long road toward resupporting Cygwin, which does
not provide siginfo_t.si_band.

It was added back in 53a578f51f21.

Reported by Vladimir Rutsky.

Patch by Mariatta Wijaya.

Also, make idlelib.idle.main() work after import idlelib.idle.