MEDIUM: flt-http-comp: Enable compression directions from algorithms

MEDIUM: flt-http-comp: Enable compression directions from algorithms

Compression options used to default to the response direction, even when
only a request algorithm was configured. Request compression then needed an
explicit direction or comp-req filter declaration. Also, the initial
direction depended on whether a filter declaration or a compression option
allocated the configuration first.

Start with no direction and let algo-req enable request compression and
algo/algo-res enable response compression. Add COMP_FL_DIR_FORCED to
remember an explicit compression direction, so that subsequent algorithm
directives do not re-enable a direction it excluded. This flag is also
inherited from defaults along with the other compression settings.

Only algorithm and direction directives enable compression implicitly.
Type, minimum-size and offload options configure its behavior but do not
enable a direction by themselves. Without an algorithm for that direction,
no compression can take place anyway.

This also avoids implicitly enabling response compression just because
request compression options were parsed before an explicit comp-req
filter. The legacy compression algo directive still enables responses.