MINOR: proxy: define proxies_list iteration functions

MINOR: proxy: define proxies_list iteration functions

Define wrappers function for <proxies_list> iteration. These functions
are used when a standard for loop over the whole list is not desirable.

The objective of this patch is to ease the conversion of <proxies_list>
from a singly linked list to a standard doubly linked list type. This
will help to keep the final conversion patch smaller.

Also, iteration over struct list is sligthly more complicated as it
requires more operation, for example for list end detection. Thus, the
wrappers will become handy to avoid tedious code repetion.