TS-3192: implement proxy.config.config_dir

proxy.config.config_dir is supposed to nominate the configuration
directory. However, it has never been implemented making it difficult
to bootstrap a Traffic Server instance into a configuration environment
that was not compiled in.

Add the RecConfigReadConfigDir() and RecConfigReadConfigPath()
management APIs to deal with correctly allowing proxy.config.config_dir
to override the compiled in layout. Apply these everywhere that
uses sysconfdir so that we get consistent behavior.

Separate default records configuration for traffic_cop. We don't
want traffic_cop to pull in all of libmgmt, but we need the default
records values so that we can correctly process environment overrides.
Split RecordsConfig.cc into data and API components so that traffic_cop
can get the raw data. Update traffic_cop to initialize its defaults
before loading records.config.

Merge environment veriable overrides into LibRecordsConfigInit()
and make sure we always call this. This solves initializatino
problems in early startup when traffic_server needs to know
configuration paths before it can sync with traffic_manager. Without
this we can only use TS_ROOT and the compiled-in layout.
Fix typo in comment