cygwin: use spawnvp for run and sh.

We revive the dead _wspawnvp code that was used on MinGW, a
platform that has not been supported for years, adapting it
for Cygwin.

* configure: Correct the _wspawnlp test; it should have been
testing for _wspawnvp function, which is what is is actually
used by the matching code. Moreover, the broken test was
calling _wspawnlp with _wspawnvp style arguments. Anyway, this
is moot because we will never detect this function in the
foreseeable future.  More importantly, adding a test for
spawnvp, which exists on Cygwin.

* stream.c: Include the <process.h> header if HAVE_WSPAWN or
HAVE_SPAWN. This was missing before; how did the _wspawnvp
call ever compile?
(w_spawnvp): New static function.
(run): spawn version now calls either _wspawwnvp or w_spawnvp
if that is not available. We test for HAVE_WSPAWN or
HAVE_SPAWN first, before HAVE_FORK_STUFF, so that we use the
spawn function preferentially. On Cygwin, we have the fork
stuff.

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