Canonicalize line endings for GPGME S/MIME encryption. (closes #3904)

This matches the behavior for S/MIME classic mode: OpenSSL converts
the line endings to cr/lf before encrypting. Although Mutt always
canonicalizes the line endings before verifying the signature, some
clients do not do this for encrypted messages.

Thanks to cooler for the patch!

crypt-gpgme.c

···
   if (!rset)
     return NULL;
 
-  plaintext = body_to_data_object (a, 0);
+  /* OpenSSL converts line endings to crlf when encrypting.  Some
+   * clients depend on this for signed+encrypted messages: they do not
+   * convert line endings between decrypting and checking the
+   * signature.  See #3904. */
+  plaintext = body_to_data_object (a, 1);
   if (!plaintext)
     {
       free_recipient_set (&rset);