MINOR: proxy: Don't close FDs if not our proxy.

MINOR: proxy: Don't close FDs if not our proxy.

When running with multiple process, if some proxies are just assigned
to some processes, the other processes will just close the file descriptors
for the listening sockets. However, we may still have to provide those
sockets when reloading, so instead we just try hard to pretend those proxies
are dead, while keeping the sockets opened.
A new global option, no-reused-socket", has been added, to restore the old
behavior of closing the sockets not bound to this process.