Exclude system.nfs4_acl xattrs, bug #475496
Stop using config["PORTDIR"] in some tests.
Support PORTAGE_REPOSITORIES environmental variable, which overrides
whole configuration of repositories (repos.conf, PORTDIR, PORTDIR_OVERLAY).
Format of PORTAGE_REPOSITORIES is identical to format of repos.conf.
Set portage._working_copy bool variable.
egencache: Support --repositories-configuration option.
Use PORTAGE_REPOSITORIES in test suite.
Move default configuration of repositories from /usr/share/portage/config/make.globals
to /usr/share/portage/config/repos.conf.
Use consistent paths of repositories in test suite and clean some code.
portageq: avoid portdbapi instantiation

The portdbapi takes time to instantiate, and for some commands we
really only need the  RepoConfig instance.
portage.settings.repositories.config_string(): Improve format of returned string.
portage.repository.config.RepoConfigLoader.config_string(): Rename a variable.
portageq: Support repositories_configuration command.
portageq repositories_configuration: Require EROOT argument.
config: add PORTDIR to expand_map

Since commit a5dd8b59fbf253a435e5ca32c5fd0d128d1d8cfb, there was no
longer a default PORTDIR setting when loading make.conf. Now, the
default main-repo location will serve as the default PORTDIR setting,
for backward compatibility.
portageq: Use uses_eroot decorator.
portageq: Delete incorrect usage of uses_eroot decorator for pquery.