gpg: Add canceled status message.

* common/status.h (STATUS_CANCELED_BY_USER): New.
* g10/passphrase.c (passphrase_to_dek): Send STATUS_CANCELED_BY_USER
instead of STATUS_MISSING_PASSPHRASE when canceled is set.

This is to prevent further pinentry tries when the pinentry is canceled
by the user during symmetric decryption.

• Signed-off-by: Ben Kibbey <bjk@luxsci.net>