Bug 4847 pt1: regression in proxy_auth ACL flags (#191)

Bug 4847 pt1: regression in proxy_auth ACL flags (#191)

r15058 "Support for --long-acl-options" in Squid-4.0.21
unintentionally removed the proxy_auth ACL support for -i/+i
flags. See bug report for details.

Fix proxy_auth ACL -i and +i flags no longer working by copying
RegexData flags registration, since ACLs for UserData all use
the same names and meanings.

Add documentation to indicate that ident and ext_user ACLs do
support -i/+i just like proxy_auth ACLs.

TODO: fix server_cert_fingerprint ACL which is still broken.