MEDIUM: unix: add preliminary support for connecting to servers over UNIX sockets

MEDIUM: unix: add preliminary support for connecting to servers over UNIX sockets

We've had everything in place for this for a while now, we just missed
the connect function for UNIX sockets. Note that in order to connect to
a UNIX socket inside a chroot, the path will have to be relative to the
chroot.

UNIX sockets connect about twice as fast as TCP sockets (or consume
about half of the CPU at the same rate). This is interesting for
internal communications between SSL processes and HTTP processes
for example, or simply to avoid allocating source ports on the
loopback.

The tcp_connect_probe() function is still used to probe a dataless
connection, but it is compatible so that's not an issue for now.

Health checks are not yet fully supported since they require a port.