MEDIUM: filters: automatically order the implicit core filters in legacy mode

MEDIUM: filters: automatically order the implicit core filters in legacy mode

In the legacy filter mode, the implicitly declared core filters (the
decompression, cache, compression and fcgi filters) are now automatically
ordered instead of relying on the user to declare them in a valid position
or reporting a configuration error.

The forced global order, used for the side-independent operations
(configuration checks, initialization and destruction), is:

  decomp-req > decomp-res > cache-store >  any other filters > http-comp-req > http-comp-res > fcgi

Two additional orders are maintained for the request and the response
channels, matching the three independent orders of the new instance mode,
and used to fill the stream's request and response filter lists and by
flt_foreach_conf() for the FLT_SIDE_REQ and FLT_SIDE_RES sides:

  * request:  decomp-req > any other filter > http-comp-res > decomp-res > http-comp-req > fcgi

  * response: fcgi > decomp-res > cache-store > any other filter > decomp-req > http-comp-res

A new FLT_CFG_FL_EXPLICIT flag marks the filters declared with the legacy
"filter" directive. Explicitly declared core filters are never moved and act
as segment boundaries: implicitly declared filters are sorted inside the
segment they appear in, so they never cross an explicitly declared one. The
request and response orders are stored in two new proxy lists,
filters_req_configs and filters_res_configs, filled with the 'by_req' and
'by_res' members of the filter configurations.

The filter configurations are now linked in the three lists at declaration
time, before flt_check() reorders them, so the lookups performed by the
rules during the configuration checks (the cache rules looking for the
cache-store filter on the response side) behave the same as before the
reordering.

Because the position of implicit filters no longer depends on the
declaration order, the errors reported when mixing an explicit filter with
implicitly declared decompression, compression, cache or fcgi filters are
removed. Explicitly declaring core filters in a conflicting order still
reports an error, the manual placement remains the user's responsibility
for explicit declarations.

The ordering was validated functionally: with an explicit filter mixed
with implicitly declared core filters, the response body is decompressed
and stored uncompressed in the cache before the trace and compression
filters see it, and a gzipped request is decompressed before reaching the
explicit filters.