MEDIUM: http: make option http_proxy automatically rewrite the URL

There are very few users of http_proxy, and all of them complain about
the same thing : the request is passed unmodified to the server (in its
proxy form), and it is not possible to fix it using reqrep rules because
http_proxy happens after.

So let's have http_proxy fix the URL it has analysed to get rid of the
scheme and the host part. This will do what users of this feature expect.
BUG/MEDIUM: http: cook_cnt() forgets to set its output type

Since comit b805f71 (MEDIUM: sample: let the cast functions set their
output type), the output type of a fetch function is automatically
considered and passed to the next converter. A bug introduced in
1.5-dev9 with commit f853c46 (MEDIUM: pattern/acl: get rid of
temp_pattern in ACLs) was revealed by this last one : the output type
remained string instead of UINT, causing the cast function to try to
cast the contents and to crash on a NULL deref.

Note: this fix was made after a careful review of all fetch functions.
A few non-trivial ones had their comments amended to clearly indicate
the output type.