negative rules + doc
Implement a "hostnames" listener option.

 listen on egress hostnames <table>

Here "table" maps IPs to hostnames.   When accepting a connection,
smtpd uses the hostname found in the mapping if there is an entry
for the local IP.  Otherwise, it uses the default server name.

While there, change the "helo" keyword to "hostnames" on relay rules.
minor documentation nit, diff by Liam J. Foy
better document the SSL bits, diff from Jason Barbier, slightly reworded
Merge branch 'master' of ssh.poolp.org:/git/opensmtpd

Conflicts:
	smtpd/parse.y
do not dereference x509 information in a mta session if we don't have a client cert