Improve config parsing of logformat definitions

Improve config parsing of logformat definitions

Squid has for some time ignored custom definitions using the same name
as internally defined formats. And overwritten custom formats when there
was a repeated definition.

* Detect logformat duplicates and produce ERROR message indicating the
  format name, config line and action taken.

* Add some missing FATAL labels on parse abort when access_log has
  multiple logformat= options configured.

* Add missing FATAL error message when logformat lines has no name
  parameter (and thus no tokens either).

* Omit the default "logformat=squid" option from cachemgr config dumps.