Windows: Fix signal numbers

Windows: Fix signal numbers

We had defined some SIG_FOO macros that appear in the code, but that are
not supported on Windows, in order to make the code compile.  But a
subsequent change will assert that a signal number is non-zero.  We now
use the signal numbers that are commonly used on POSIX systems.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>