make gpgme usage simpler

This is probably my very own mistake, but I only just noticed https://dev.mutt.org/doc/manual.html#crypt-use-gpgme after many years of compiling Mutt against gpgme. I only recently disabled pgp support, because gpgme should be able to do gpg (and smime), and noticed that I could no longer send signed/encrypted messages. Pressing 'y' didn't result in any message or action in the send screen. Only when I removed signing/encryption it would actually send my message. After setting crypt_use_gpgme=yes in my .muttrc sending such message worked.

I'm not sure what the rationale behind crypt_use_gpgme is or was, but it feels kind of weird that it defaults to no. When gpgme support is compiled out, it is obvious it can't use gpgme, but if it is, then I feel it's much more obvious for a user to have to disable using gpg if classic pgp is compiled in at all rather than to enable it.

In any case, a message/error of some sort to hint that no crypt backend was compiled for the currently selected mode in the send screen (when pressing 'y') would be nice.