MINOR: config: define wrapper for proxies loop during check config

MINOR: config: define wrapper for proxies loop during check config

On check_config_validity(), proxy post init code is reexcuted over
several proxies list : the main proxies list, log sections and sinks.

This patch defines an internal wrapper to help performing this
iteration. It will become necessary once each list is converted from a
singly linked format to a doubly linked struct list. Once all lists are
converted, the wrapper should be removed as it will be once again
unnecessary.