Cleanup effort in prep for GA push: Trim trailing whitespace... no func change

Fix MPM event/libserf build problems: If libserf was installed during build, MPM event was picking up the headers but not linking with libserf, causing unresolved symbol errors. Now build MPM event with or without serf support depending on --with-serf being specified or not.

prepare to add mod_serf to NetWare build.

If the MPM supports serf in its core, use this to drive the proxied request completely async from the original thread. Right now it uses a 100ms timer to determine when the request is actaully complete, but I think this can be removed in the long run with a better mechanism.

Work in Progress. Add Clustered proxying support to mod_serf, by using the heartbeats system. No preconfiguration of cluster members is needed. Just a config like this: SerfCluster sweet heartbeat file=/var/cache/apache/hb.dat SerfCluster sour heartbeat file=/var/cache/apache/cluster2.dat <Location "/"> SerfPass cluster://sweet </Location> <Location "/different_cluster"> SerfPass cluster://sour </Location> The location of all possible destination servers is provided by a new providers interface, that includes configuration checking of the arguments to the SerfCluster command, solving one of the worst problems with the mod_proxy load balancer subsystem.