REORG: filters: Prepare creation of the HTTP compression filter

REORG: filters: Prepare creation of the HTTP compression filter

HTTP compression will be moved in a true filter. To prepare the ground, some
functions have been moved in a dedicated file. Idea is to keep everything about
compression algos in compression.c and everything related to the filtering in
flt_http_comp.c.

For now, a header has been added to help during the transition. It will be
removed later.

Unused empty ACL keyword list was removed. The "compression" keyword
parser was moved from cfgparse.c to flt_http_comp.c.