[MINOR] acl: detect and report potential mistakes in ACLs

[MINOR] acl: detect and report potential mistakes in ACLs

I've discovered a configuration with lots of occurrences of the
following :

    acl xxx hdr_beg (host) xxx

The problem is that hdr_beg will match every header against patterns
(host) and xxx due to the space between both, which certainly is not
what the user wanted. Now we detect such ACLs and report a warning
with a suggestion to add "--" between "hdr_beg" and "(host)" if this
is definitely what is wanted.