Use a case-sensitive version of SafeConfigParser everywhere
This change has the potential to break existing setups, but the current
behaviour (the keys in configuration files are always lower-cased) can
bite us in a few places:
- no way to use a Command in [defaults]
- hgext.Extension doesn't work in [extensions]
- you can't use an Upper/case/PATH in the [paths] section of hgweb.config
- you can't (easily) protect paths with upper-case letters with the
acl extension
- you can't specify a /Path/TO/a/rEPO in the [reposubs] section for
the notify extension