Split ACL.{cc,h} and src/acl_noncore.cc into acl/Acl and acl/Gadgets, moving
high-level global functions into Gadgets and leaving basic API types in Acl.
Moved horrific acl_access::containsPURGE into aclPurgeMethodInUse to avoid
exposing basic ACL API to "strategy" templates and HTTP-specific PURGE method.
The aclPurgeMethodInUse global lives in acl/libacls, which is a top-level
library that already contains a lot of data-specific code.