[MEDIUM] proxy: add a PAUSED state to listeners and move socket tricks out of proxy.c

[MEDIUM] proxy: add a PAUSED state to listeners and move socket tricks out of proxy.c

Managing listeners state is difficult because they have their own state
and can at the same time have theirs dictated by their proxy. The pause
is not done properly, as the proxy code is fiddling with sockets. By
introducing new functions such as pause_listener()/resume_listener(), we
make it a bit more obvious how/when they're supposed to be used. The
listen_proxies() function was also renamed to resume_proxies() since
it's only used for pause/resume.

This patch is the first in a series aiming at getting rid of the maintain_proxies
mess. In the end, proxies should not call enable_listener()/disable_listener()
anymore.