Gpg4win expects native Windows paths even when it's being run from a
cygwin environment. In order to satisfy that, update `s:shellescape` to
know whether it should convert the paths to native Windows paths using
`cygpath -am`.
In order to detect whether cygpath should be invoked, look for the
combination of `has('win32unix')` being true and `gpg --version`
reporting a Windows style path for gpg's home directory.
Closesjamessan/vim-gnupg#62
Signed-off-by: James McCoy <jamessan@jamessan.com>