Cygwin: redefine CW_CMDLINE to CW_CMDLINE_ALLOC

Cygwin: redefine CW_CMDLINE to CW_CMDLINE_ALLOC

Make sure to

- append a trailing \0 as with Windows multistrings, so the end of
  the string can be recognized by the caller, and

- allocate cmdline on the user heap so the caller can free the
  multistring after usage.

Fixes: 831d6fa520ed4 ("* external.cc (cygwin_internal): Implement CW_CMDLINE.")
Signed-off-by: Corinna Vinschen <corinna@vinschen.de>