MINOR: log: convert list to standard doubly linked one

MINOR: log: convert list to standard doubly linked one

Log-forward proxies are not stored in <proxies_list> alongside other
visible user proxies. Instead, they are stored in a dedicated list
<cfg_log_forward>.

As with the visible proxies list, log forwarders list was defined as a
simple singly linked one. This patch converts it to the doubly linked
standard list type, frequently used in haproxy source code. Proxy <el>
member is reused as list attach point. It was only used by defaults
instances prior to this patch.

This patch does not bring noticeable change. However it will simplify
visible proxies list similar conversion as both lists are looped over
together during check_config_validity().