app-portage/eclass-manpages: Introduce additional variable classes

app-portage/eclass-manpages: Introduce additional variable classes

Add a few additional variable classes to better emphasize the specifics
of different kinds of variables set for eclasses, and by eclasses.

The change applied, each eclass variable can belong to one of
the following five eclasses:

1. (default) - variable set by ebuild, influences eclass behavior.

2. @PRE_INHERIT - likewise but must be set above the inherit line,
and not modified afterwards.

3. @USER_VARIABLE - variable to be set by user (make.conf), and not
by ebuilds. This mostly involves MAKEOPTS-style variables.

4. @OUTPUT_VARIABLE - variable that is generated and defined by eclass,
and ebuilds can *read* it.

5. @INTERNAL - (existing) internal use variable.