[MEDIUM] Implement "track [<backend>/]<server>"

[MEDIUM] Implement "track [<backend>/]<server>"

This patch implements ability to set the current state of one server
by tracking another one. It:
 - adds two variables: *tracknext, *tracked to struct server
 - implements findserver(), similar to findproxy()
 - adds "track" keyword accepting both "proxy/server" and "server" (assuming current proxy)
 - verifies if both checks and tracking is not enabled at the same time
 - changes set_server_down() to notify tracking server
 - creates set_server_up(), set_server_disabled(), set_server_enabled() by
   moving the code from process_chk() and adding notifications
 - changes stats to show a name of tracked server instead of Chk/Dwn/Dwntime(html)
   or by adding new variable (csv)

Changes from the previuos version:
 - it is possibile to track independently of the declaration order
 - one extra comma bug is fixed
 - new condition to check if there is no disable-on-404 inconsistency