Improve documentation of repos.conf.
Sanitize PYTHONPATH for helpers, bug #477764
Split PORTDIR_OVERLAY with shlex.
inherit: optimize eclass search

Use PORTAGE_ECLASS_LOCATIONS variable ordered so that we can break out
of the eclass search loop as soon as the first match is found.
emerge-webrsync: use PORTAGE_REPOSITORIES var

Using this variable allows us to get all required configuration data
with a singe portageq envvar call.
make.conf.5: note usersync reqs for bug #277970
Quote ${PORTAGE_ECLASS_LOCATIONS[@]}
Add support for multiple repositories in `emerge --sync`.

Configuration of synchronization is specified by new attributes
supported in repos.conf: sync-cvs-repo, sync-type, sync-uri.

SYNC variable is no longer supported, since it could not override
gentoo.sync-uri attribute now set in default configuration of
repositories (/usr/share/portage/config/repos.conf).
Move hardcoded atoms of cvs, git and rsync to portage.const module and fix category of git atom.