lintian{,-info}: Add --[no-]user-dirs cmd option

Add new command line option to disable loading data, profiles and
config files from "user directories" ($HOME and /etc).  This option
can be used to replace the (private) LINTIAN_INTERNAL_TESTSUITE
environment variable.

It is mostly useful cases where the user's setting should not affect
the Lintian results (e.g. testsuites).

Signed-off-by: Niels Thykier <niels@thykier.net>