md5, sha1, sha256, sha512: support mandating use of openssl

md5, sha1, sha256, sha512: support mandating use of openssl

* m4/gl-openssl.m4 (gl_CRYPTO_CHECK): Adjust the --with-openssl
description, to list the now 3 separate options.  Also don't
mention the default=no, since this is implicit given the option
is described as --with-openssl rather than --without-openssl.
If projects change the default they're free to document that.
With --with-openssl[=yes] we now error out when the specified
hash algorithm is not available in libcrypto.