MINOR/CLEANUP: proxy: rename "proxy" to "proxies_list"

MINOR/CLEANUP: proxy: rename "proxy" to "proxies_list"

Rename the global variable "proxy" to "proxies_list".
There's been multiple proxies in haproxy for quite some time, and "proxy"
is a potential source of bugs, a number of functions have a "proxy" argument,
and some code used "proxy" when it really meant "px" or "curproxy". It worked
by pure luck, because it usually happened while parsing the config, and thus
"proxy" pointed to the currently parsed proxy, but we should probably not
rely on this.

[wt: some of these are definitely fixes that are worth backporting]