Rename MOUNTCRITLOCAL to CRITLOCALMOUNTED to avoid a name collision
on case insensitive file systems

Make it work with empty /var

Split the local disk availability step into two phases to allow scripts
that pre-populate parts of the system (e.g. a tmpfs based /var) an
easy place to plug in like:

# REQUIRE: mountcritlocal
# BEFORE: MOUNTCRITLOCAL

This also cleans up the existing special handling a bit by separating it
into new scripts. All later scripts now depend on MOUNTCRITLOCAL.
Discussed on tech-userlevel some time ago.