server priorities

hello,

I recently came across the problem of server priorities: I needed to
assign priority levels to servers and make pen's backend selection
algorithm consider these priority values.
Attached is a patch that tries to solve this priority issue.
Now when pen needs to select the best server for a new client, it will
select a server with the highest priority and the smallest load (of the
available servers). Priorities might be combined with weight.
Use the '-I' command line option to make use of priorities. An example
pen.cfg with priorities:

server 0 address server1 port 80 weight 2 priority 0
server 1 address server2 port 80 weight 1 priority 1

Balint