squidclient: --ping mode module support

squidclient: --ping mode module support

  Module support:

Update squidclient support modules with different logics
and configuration option sets as a basis for multiple
protocol support.

A mechanism is added to allow each module to have its own
command line option set. Any option unknown to the current
module handler drops back to the main loop for processing.

  --ping mode module:

Break the existing code "ping mode" operations and command
line processing out from the main squidclient.cc into Ping.*

Ping-specific short command line options are now only parsed
after a mode flag (--ping) is presented. This frees up the
-g and -I options for use by other non-ping modules in future.

Also, shuffle squidclient code into its own directory
tools/squidclient/ to keep the tool code files clearly
identifiable now that they are multiplying.