signal, exec, wait, pid: improvements, especially to exec

signal, exec, wait, pid: improvements, especially to exec

- fix popen [open "|pipeline ..."] to return meaningful status in close
  (but note that stderr is not captured)
- popen pipelines can now be used as the target of exec redirection
- overally improvements to exec on windows. Now crt file descriptors
  are used throughout
- add support for [pid], [wait] and popen on windows
- os.wait is now wait, and integrates with [exec ... &] to be able
  to wait for running background tasks
- [socket pipe] is now also [pipe] and is supported on windows
- [file tempfile] is supported on windows
- move duplicated code between jim-aio.c and jim-exec.c to jimiocompat.c
- Fix [exec] on windows to match unix semantics wrt sharing the parent stream
  unless redirected rather than using /dev/null
- On windows redirect to or from /dev/null is automatically converted to NUL:
- If signal support is disabled, implement a minimal Jim_SignalId() for exec and wait
- aio now supports getfd, to return the underlying file descriptor.
  This is used by exec to support redirection, and allows popen channels
  to support exec redirection.

Signed-off-by: Steve Bennett <steveb@workware.net.au>