Use a default "IncludeDir" only when no config file was specified

Use a default "IncludeDir" only when no config file was specified

No longer use a default built-in value for the "IncludeDir" directive
when a configuration file was explicitly specified on the command line
using "--config"/"-f": This way no default include directory is scanned
when a possibly non-default configuration file is used which
(intentionally) did not specify an "IncludeDir" directive.

With this patch you now can use "-f /dev/null" for checking all built-in
defaults, regardless of any local configuration files in the default
drop-in directory (which would have been read in until this change).