MEDIUM: Add helper for agent check events.

Break agent check handling out of event_srv_chk_r().

This is in preparation for supporting agent check results
returned in an HTTP header.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Parser to allow matching of HTTP header

Replace the current header parser, which simply skips the headers,
with a version that allows matching of a key in a header.

This is in preparation for supporting agent check results
returned in an HTTP header.

Signed-off-by: Simon Horman <horms@verge.net.au>
MEDIUM: Add http-check agent-hdr option

Allow agent checks to obtain information in
an HTTP header of the response to an http check.

Signed-off-by: Simon Horman <horms@verge.net.au>