Mention the setcap solution to allow listening to port 443 and 80.

Explain custom configuration files.

Avoid Nonetype-related errors.

TODO: Would be better to sanitize every parameter at beginning.

our specified revision seems to be disappeared maybe by force push.

Old versions of slapproxy/slapos master don't know it.
Catching it here so that we have a chance to debug problems in SRs.

Tests directory of inserted paths also needs to be in sys.path so that tests
can import other tests.