Ban acl key changes in req_header, rep_header, and note ACLs (#1210)

Ban acl key changes in req_header, rep_header, and note ACLs (#1210)

* req_header and rep_header ACLs; the key is the header-name argument:

    acl barred req_header X-1 bar
    acl barred req_header X-2 bar # never matches since commit a0b240c
    http_access deny barred

* the "note" ACL; the key is the annotation name argument:

    acl banned note color green  # never matches since commit 39baccc
    acl banned note weight heavy
    http_access deny banned

Squid did write a cache.log ERROR for req_header/rep_header key changes
but was silent about the preceding "note" ACL rules being ineffective
after a key change (e.g., Squid was not denying "green" requests above).

Now, Squid rejects configurations containing such ACL key changes.

Also significantly reduced the corresponding parsing code duplication.