This patch is based on Christian Brabandt's patch sent
to mutt-users.

Add two new configuration variables: $postpone_encrypt and
$postpone_encrypt_as.  When $postpone_encrypt is set and a message is
marked for encryption, the message will be encrypted using the key
specified in $postpone_encrypt_as before saving the message.

In this patch, $postpone_encrypt_as must be specified.  I experimented
with passing safe_strdup( NONULL (PostponeEncryptAs)) when unspecified,
but although gpg.conf has a default-key setting, I could not get it to
work properly. (pgpclassic gave an error message and gpgme sefaulted.)

Although not necessary, this patch turns off signing during encryption
of the postponed message (and turns it back on before saving), since
there is no need to sign the message yet.