lintian: Add --ignore-lintian-env cmd line option

Add a new command line option ("--ignore-lintian-env") to make lintian
ignore all of environment variables matching "LINTIAN_*".  Other
environment variables like TMPDIR and DEB_VENDOR are unaffected by
this.

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