MINOR: log: use curproxy during config parsing

MINOR: log: use curproxy during config parsing

Adjusts configuration parsing for log-forward sections : replace
<cfg_log_forward> head pointer list usage in individual keywords parser
by <curproxy> global variable. This parsing patterm is already
implemetned in cfgparse-listen module.

This is only a refactoring change without any visible effect. It is a
preparatoy step before converting <cfg_log_forward> to a full doubly
linked struct list.