Try to contain the failure when we are running without socketpair().

Try to contain the failure when we are running without socketpair().

Some win32 systems (mostly those using Kaspersky, it would seem)
prevent us from faking socketpair().  This makes our signal
notification code just not work.  Our response since 1.4 has been to
assert.  For users who would rather work without signals than not work
at all, this has been a regression from 1.3e.

This patch makes adding signal events fail in this case; there's no
reason to kill the whole process.

svn:r1303