squidclient: send cachemgr password via -w option

squidclient: send cachemgr password via -w option

Preparation for internal cachemgr updates to use real proxy-auth.

The cachamgr password may now be sent in three ways:
  Deprecated:      mgr:info@password
  Current Option:  -w password mgr:info
  Preferred:       -u username -w password mgr:info

The old explicit @ syntax is now deprecated for visible use. The background
systems will still send it that way for cache_object: URLs. Use of this
overrides any -w option set. So it is still possible to login to a proxy
with one set of credentials and pass a separate password to the cachemgr.
The long-term plan is to drop @ completely in future.

The current option of just -w will convert the password to @ syntax in the
background but not add Proxy-Authentication headers. This may die in future.

The preferred alternative is to use -u and -w which triggers addition of real
Proxy-Authenticate headers. The username is not yet used by cachemgr but
may be required by the proxy ACL configuration.