[gentoo-alt] prefix chaining

[Markus Duft]

Hi!

i'm done working on my prefix-chaining project for now. i have
(attached) patches ready for commit (and 2 new packages, but that should
not matter to anyone anyway :))

the one is a newer version of the portage patch i submitted lately, and
the other one patches baselayout, so that prefix chaining can work.

for all these things i can say: they won't hurt anybody, as long as
there is no READONLY_EROOT set in make.conf, which should not be the
case ;) that's the trigger that activates all changes.

if there is no READONLY_EROOT, behaviour of all parts of portage and
baselayout should be exactly the same.

ok to commit? maybe we could think about checking the portage patch into
the prefix-portage svn tree?

since i haven't commented on the baselayout patch yet, i'll do it here
(the portage patch is basically the same as the one i submitted lately,
with small improvements):

etc/env.d/00basic:
	* added EPREFIX variable, since if the currently used portage
	  instance comes from a chained prefix, portage needs to be
	  explicitly informed of the EPREFIX. this should not disturb
	  anybody, since it can be overridden after starting the prefix
	  again...
etc/profile:
	* the profile now sources all profiles (recursively) of all
	  parent prefixes in the chain.
	* if we are in a chained environment, some variables need
	  speacial treatment, since i want some of the chained envs to
	  be there in the current prefix. (PATH, MANPATH and in some
	  cases PKG_CONFIG_PATH. others may come...)
	* if we are in a chained environment, don't append /usr/.. and
	  /... paths to PATH, unless we're the top level prefix in the
	  chain (which behaves the same as if there where no chain).
	* the last hunk is for correctly setting the chained variables
	  which i listed above (PATH, MANPATH, etc.).

Questions?

If nobody has objections, i'll commit. i'll wait a few hours before
committing.

Cheers, Markus