Fork into a monitor parent process and an actual proxy child process,
communicating over AF_UNIX sockets. Certain privileged operations are
performed through the privileged parent process, like opening log files
or listener sockets, while all other operations happen in the child
process, which can now drop its privileges without side-effects for
log file opening and other privileged operations. This is also a
preparation for -l/-L logfile reopening through SIGUSR1.
This means that -S and -F are no longer relative to chroot() if used
with -j. This is a deliberate POLA violation.
