Automated squid.conf parser testing with squid -k parse

Automated squid.conf parser testing with squid -k parse

This make the build process testing run 'squid -k parse' with a series
of test configuration files to exercise edge cases and backward
compatibility.

The default squid.conf which is to be installed on new systems is always
tested.

Other config files with unit tests can be dropped into
test-suite/squidconf/ and will be parse tested automatically.