Simplify Cygwin checks, part 1

Because OPENSSL_SYS_CYGWIN will keep OPENSSL_SYS_UNIX defined, there's
no point having checks of this form:

    #if (defined(OPENSSL_SYS_UNIX) || defined(OPENSSL_SYS_CYGWIN))

Reviewed-by: Andy Polyakov <appro@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/5060)