Implement getentropy for Cygwin

Implement getentropy for Cygwin

	* miscfuncs.cc (getentropy): Move fhandler_dev_random::crypt_gen_random
	here and rename to getentropy.  Fix type and return values to match
	getentropy requirements.
	* miscfuncs.h (getentropy): Add prototype.
	* fhandler.h (fhandler_dev_random::crypt_gen_random): Remove prototype.
	* fhandler_random.cc (fhandler_dev_random::crypt_gen_random): Drop.
	(fhandler_dev_random::write): Use getentropy instead.
	(fhandler_dev_random::read): Ditto.
	* fhandler_socket.cc (fhandler_socket::af_local_set_secret): Ditto.

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