Cygwin: make sure exec'ed process exists early in process list

Cygwin: make sure exec'ed process exists early in process list

killpg(pgid, 0) (or kill_pgrp(pgid, si_signo=0), in signal.cc)
fails (returns -1) even when there is a process in the process
group pgid, if the process is in the middle of spawnve(), see

  https://cygwin.com/pipermail/cygwin/2022-May/251479.html

When exec'ing a process the assumption is that the exec'ed process creates its
own symlink (in pinfo::thisproc() in pinfo.cc). If the exec'ing process
calls NtClose on it's own winpid symlink, but the exec'ed process didn't
progress enough into initialization, there's a slim chance that neither
the exec'ing process, nor the exec'ed process has a winpid symlink
attached.

Always create the winpid symlink in spawn.cc, even for exec'ed Cygwin
processes.  Make sure to dup the handle into the new process, and stop
creating the winpid symlink in exec'ed processes.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>