Manual page: clarify that the --address and --ipset options take one or
more domains rather than just two. Clarify that --ipset puts addresses
in all ipsets, it is not a 1:1 mapping from addresses.

Also increase the width for options output in --help, some options were
truncated leading to confusing output. Almost all options and
descriptions are now within the 120 colums limit.

(--server=1.2.3.4@eth0) when the named interface
is destroyed and recreated in the kernel.

This only makes sense for interfaces which we're configured to listen on.
Thanks to Andrew Shadura for bug report.

whereby receipt of SIGHUP would erroneously engage timestamp checking.

This could cause dnsmasq to enter a tight loop on systems
with a very large number of network interfaces.

The check that there's enough space to store the DHCP agent-id
at the end of the packet could succeed when it should fail
if the END option is in either of the oprion-overload areas.
That could overwrite legit options in the request and cause
bad behaviour. It's highly unlikely that any sane DHCP client
would trigger this bug, and it's never been seen, but this
fixes the problem.

Also fix off-by-one in bounds checking of option processing.
Worst case scenario on that is a read one byte beyond the
end off a buffer with a crafted packet, and maybe therefore
a SIGV crash if the memory after the buffer is not mapped.

Thanks to Timothy Becker for spotting these.

Thanks to Eric Westbrook for spotting this.

Thanks to Jan Psota.