MEDIUM: http: emulate "block" rules using "http-request" rules

MEDIUM: http: emulate "block" rules using "http-request" rules

The "block" rules are redundant with http-request rules because they
are performed immediately before and do exactly the same thing as
"http-request deny". Moreover, this duplication has led to a few
minor stats accounting issues fixed lately.

Instead of keeping the two rule sets, we now build a list of "block"
rules that we compile as "http-request block" and that we later insert
at the beginning of the "http-request" rules.

The only user-visible change is that in case of a parsing error, the
config parser will now report "http-request block rule" instead of
"blocking condition".