MAJOR: acl: ensure that implicit table and proxies are valid

MAJOR: acl: ensure that implicit table and proxies are valid

A large number of ACLs make use of frontend, backend or table names in their
arguments, and fall back to the current proxy when no argument is passed. If
the expected capability is not available, the ACL silently fails at runtime.

Now we make all those names mandatory in the parser and we rely on
acl_find_targets() to replace the missing names with the holding proxy,
then to perform the appropriate tests, and to reject errors at parsing
time.

It is possible that some faulty configurations will get rejected from now
on, while they used to silently fail till now. This is the reason why this
change is marked as MAJOR.