This fix (across 4 files in 3 directories) solves a subtle problem with

signal handlers in a fork()ed child process when Python is compiled with
thread support.  The bug was reported by Scott <scott@chronis.icgroup.com>.

What happens is that after a fork(), the variables used by the signal
module to determine whether this is the main thread or not are bogus,
and it decides that no thread is the main thread, so no signals will
be delivered.

The solution is the addition of PyOS_AfterFork(), which fixes the signal
module's variables.  A dummy version of the function is present in the
intrcheck.c source file which is linked when the signal module is not
used.