Implement process lockfile and run user/group

- sets effective UID, typically root
- grabs write lock on server/lockfile (e.g. /var/run/zbroker.pid)
- dumps PID into lockfile
- sets group and/or user from server/group, server/user
- else sets back to real UID

Note, does not work using normal 'make' as zbroker isn't the real binary.
Here is how I build/test:

    MAIN=zbroker
    sudo rm $MAIN
    gcc -g -o $MAIN \
        $MAIN.c \
        zpipes_msg.c zpipes_client.c zpipes_server.c \
        -lczmq -lzmq -lzyre -luuid -lsodium
    test $? -ne 0 && exit

    sudo chown root $MAIN
    sudo chgrp root $MAIN
    sudo chmod +s $MAIN
    ./$MAIN

Note example of zbroker.cfg:

    server
        timeout = 10        #   Client connection timeout
        background = 0      #   Run as background process
        workdir = .         #   Working directory for daemon
        animate = 1         #   Do we animate or not?
        user = ph           #   Run as this user
        group = ph          #   Run as this group
        lockfile = /var/run/zbroker.pid

Fixes #52
Fixes #53