Simplify `openStream' and `session'; allow for SockAddr; validate IP

`openStream' and `session' now takes a `HostName'. If the hostname is
an IP adress, we connect to it using the default XMPP port. If it's a
hostname, we perform the SRV lookup (or fallback on A/AAAA lookups and
the default XMPP port).

This patch allows for the use of a socket and socket address pair in
the settings, giving advanced users additionally flexibility. This
field can also be used by users that want to use a non-standard port
in combination with a) a misconfigured XMPP domain name (one without
SRV records), or b) connection by IP.

The "manual" A(AAA) lookups have been kept in order to allow the
Pontarius XMPP client to attempt to connect to multiple IP addresses.