MAJOR: lua: Forbid calls to Channel functions for LUA scripts in HTTP proxies

MAJOR: lua: Forbid calls to Channel functions for LUA scripts in HTTP proxies

Functions from then Channel class are now forbidden for LUA scripts called from
HTTP proxies. These functions totally hijacked the HTTP parser, leaving it in an
undefined state. This patch is tagged as MAJOR because it could be see as a
compatibility breakage. But a LUA script using one of these functions has a very
low probablity to work correctly except by chance.

So, concretely, following functions are concerned: Channel.get, Channel.dup,
Channel.getline, Channel.set, Channel.append, Channel.send,
Channel.forward. Others remain available.