sinksmtp: add support for matching the remote IP against CIDRs et al

You can also put the list of IPs and CIDRs to match against into a file.
sinksmtp: improve rules parsing error message in a confusing case

'reject host something or bogus bogus' used to complain that the
right side of the OR was empty. This is technically correct but
very misleading to read; the right side of the OR is empty not
because we ran out of things to parse but because 'bogus' is not
a match operator. Now we generate the right error so I will be
less confused.
sinksmtp: make 'ehlo PATTERN' a synonym for 'helo PATTERN' in rules

I just wrote two rules using 'ehlo PATTERN' instead of 'helo PATTERN'
so my mind is trying to tell me something about how I think the rules
language should work. Now sinksmtp supports that because really, why
not.

This implies that we may grow support for 'ehlo-has' at some point.
But that's for later (if I make that particular mistake too).