MINOR: proto-tcp: make use of connect(AF_UNSPEC) for the pause

MINOR: proto-tcp: make use of connect(AF_UNSPEC) for the pause

Currently the suspend/resume mechanism for listeners only works on Linux
and we resort to a number of tricks involving shutdown+listen+shutdown
to try to detect failures on other operating systems that do not support
it. But on Linux connect(AF_UNSPEC) also works pretty well and is much
cleaner. It still doesn't work on other operating systems but the error
is easier to detect and appears safer. So let's switch to this.