hgweb.cgi and hgwebdir.cgi fixes/cleanups for user configurable parts:
- "import os" was missing in hgwebdir.cgi, added to comments for both script.
- HGENCODING has to be set before importing hgweb, otherwise you'll get
mixed encoding in changelog vs. file content/diffs.
- "import sys" is only needed when changing sys.path
- document purpose of cgitb.