vconn: Factor out common code from TCP and SSL vconns.

vconn: Factor out common code from TCP and SSL vconns.

The TCP and SSL vconn implementations had a lot of common code to make
and accept TCP connections, which this commit factors out into common
functions in socket-util.c.

Also adds the ability to bind ptcp and pssl vconns to a particular IP
address instead of the wildcard address.