[perl #85104] TODO test for preserving $^E across signal handlers

and tests Win32 signal emulation too
[perl #85104] work harder to save error numbers

Updates the errno save macros to save the platform error number where
needed for more than just VMS.

The OS/2 code is untested.
[perl #85104] save platform specific error code across signal handlers

This was previously done for VMS, add it (tested) for Win32 and
(untested) for OS/2.