MINOR: http: add a global setting to decide on warnings for http rules

MINOR: http: add a global setting to decide on warnings for http rules

The default choice of "notice" as level to emit a warning for an
http-request rule relying on unreliable processing may not always be
the best choice. This commit adds a new global "on-unreliable-rules"
directives which takes 3 parameters, "notice" (the default), "warn"
and "ignore". Choosing "warn" will emit a warning instead of a notice,
so that a config using "zero-warning" will detect them. This might be
helpful after a config was sanitized in order to make sure no new
warnings are introduced. And "ignore" may be used to silence all
messages when migrating a config.