Fix exec migration on Windows (w32+w64).

Fix exec migration on Windows (w32+w64).

* Use cmd instead of /bin/sh on Windows.

* Try to auto-detect cmd.exe's path, but default to a hard-coded path.

Note that this will require that gspawn-win[32|64]-helper.exe and
gspawn-win[32|64]-helper-console.exe are included in the Windows binary
distributions (cc: Stefan Weil).

Signed-off-by: "John Berberian, Jr" <jeb.study@gmail.com>
Reviewed-by: Juan Quintela <quintela@redhat.com>
Signed-off-by: Juan Quintela <quintela@redhat.com>