Function:

With the fold acl flag enabled (default: disabled, keeping the
existing behavior), ACLs are optimized in that subnets contained in
other entries are skipped (e.g. if 1.2.3.0/24 is part of the ACL, an
entry for 1.2.3.128/25 will not be added) and adjacent entries get
folded (e.g. if both 1.2.3.0/25 and 1.2.3.128/25 are added, they will
be folded to 1.2.3.0/24).

Skip and fold operations on VCL entries are output as warnings during
VCL compilation as entries from the VCL are processed in order.

Logging under the VCL_acl tag can change with this parameter enabled:
Matches on skipped subnet entries are now logged as matches on the
respective supernet entry. Matches on folded entries are logged with a
shorter netmask which might not be contained in the original ACL as
defined in VCL. Such log entries are marked by "fixed: folded".

Negated ACL entries are excluded from folds.

Implementation:

The sort functions are changed such that the previous semantics are
preserved: negative return values signify "a < b", positive return
values signify "a > b". But additionally the values -2/2 and -3/3 are
introduced (and given enums) to signify "contained in supernet" and
"directly adjacent to". This allows for mostly unchanged code with
vcc_acl_fold disabled.

For the "subnet contained in supernet" case, all contained subnets are
removed. By sort order, caontained subnets are always to be found left
of supernets.

For the "fold adjacent" case, the netmask of the entry with the
smaller network number is decreased by one and the other entry
removed. Because changing the netmask might affect sort order, we
reinsert the changed entry.