MINOR: payload/config: Warn if a L6 sample fetch is used from an HTTP proxy

MINOR: payload/config: Warn if a L6 sample fetch is used from an HTTP proxy

L6 sample fetches are now ignored when called from an HTTP proxy. Thus, a
warning is emitted during the startup if such usage is detected. It is true
for most ACLs and for log-format strings. Unfortunately, it is a bit painful
to do so for sample expressions.

This patch relies on the commit "MINOR: action: Use a generic function to
check validity of an action rule list".