allow presence signing to be disabled
The attached patch allows presence signing to be disabled on a per account basis without having to disable GPG completely, ie. en- and decrypting messages if needed is still possible without being bothered by the passphrase dialog on status change including returning from auto-away.
I added get_signed_presence() so that get_signed_msg stays as general as its name implies and could still be used for signing other things than a presence.