Add a subject header on armored messages

Spam filters usually adversely score messages that don't have a Subject
header, even though it's not an RFC2822 MUST.
Relax chain criteria for randhops and dummies
New outbound pool format and relaxed chains

The new outbound pool format doesn't contain recipient fields.
Recipients are now extracted from the To and Cc headers at mailing time.

Relaxed chains means randhops will use any remailer they can, if none
match the Latency/Uptime criteria.  This reduces the risk of lost
messages.