MEDIUM: http: add actions "replace-header" and "replace-values" in http-req/resp

MEDIUM: http: add actions "replace-header" and "replace-values" in http-req/resp

This patch adds two new actions to http-request and http-response rulesets :
  - replace-header : replace a whole header line, suited for headers
                     which might contain commas
  - replace-value  : replace a single header value, suited for headers
                     defined as lists.

The match consists in a regex, and the replacement string takes a log-format
and supports back-references.