Don't use deprecated dllwrap on Cygwin.

The preferred method is to use "cc -shared", and this allows binaries
to be rebased if required, unlike dllwrap.

Backpatch to 9.0 where we have buildfarm coverage.

There are still some issues with Cygwin, especially modern Cygwin, but
this helps us get closer to good support.

Marco Atzeri.
Fix some more bugs in signal handlers and process shutdown logic.

WalSndKill was doing things exactly backwards: it should first clear
MyWalSnd (to stop signal handlers from touching MyWalSnd->latch),
then disown the latch, and only then mark the WalSnd struct unused by
clearing its pid field.

Also, WalRcvSigUsr1Handler and worker_spi_sighup failed to preserve
errno, which is surely a requirement for any signal handler.

Per discussion of recent buildfarm failures.  Back-patch as far
as the relevant code exists.