Support %un (any available user name) format code for external ACLs.

Support %un (any available user name) format code for external ACLs.

The same %un code, with the same meaning is already supported in access.log.
In an external ACL request, it expands to the first available user name
from the following list of information sources:

  - authenticated user name, like %ul or %LOGIN
  - user name supplied by an external ACL to Squid via the "user=..."
    key=value pair, like %ue or %EXT_USER
  - SSL client name, like %us
  - ident user name, like %ui

Based on Amos Jeffries 2011 patch and "arronax28" design:
http://www.squid-cache.org/mail-archive/squid-dev/201112/0080.html
with TODO completion by Measurement Factory