BUG/MEDIUM: lua: Lua applets must not fetch samples using http_txn

BUG/MEDIUM: lua: Lua applets must not fetch samples using http_txn

If a sample fetch needing http_txn is called from an HTTP Lua applet,
the result will be invalid and may even cause a crash because some HTTP
data can be forwarded and the HTTP txn is no longer valid.

Here the solution is to ensure that a fetch called from Lua never
needs http_txn. This is done thanks to a new flag HLUA_F_MAY_USE_HTTP
which indicates whether or not it is safe to call a fetch which needs
HTTP.

This fix needs to be backported to 1.6.