Update sys.path in scripts only when using not installed instance of Portage.

Modules of installed instance of Portage are now placed in standard location,
which is included in default sys.path.
chpathtool: migrate docstrings to a variable

Due to python optimizations, __doc__ are removed.
Reviewed by: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
xattr-helper: Migrate docstrings to a variable

Due to python optimizations, __doc__ are removed.
Reviewed by: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
portageq: Migrate docstrings to a dictionary

Due to python optimizations, __doc__ are removed.
Reviewed by: Arfrever Frehtes Taifersar Arahesis <Arfrever@Apache.Org>
testpath: Add a generic testpath script

Source this testpath script from the base checkout directory to run all checkout code.
Revert "Rewrite default ebuild phase setting code" (bug 523182)

This reverts commit 772ed29fd9e7cf722aed943adbe33a27f250e1ff.
X-Gentoo-Bug: 523182
X-Gentoo-Url: https://bugs.gentoo.org/show_bug.cgi?id=523182
_quickpkg_dblink: fix bug #523152

This fixes the unmerge-backup and downgrade-backup features to be
compatible with the new setup.py quickpkg install location, while
preserving compatibility for running from a source tree (for unit tests
or developement purposes). If not running from a source tree, then the
PATH variable is used to locate the quickpkg binary.

X-Gentoo-Bug: 523152
X-Gentoo-Bug-URL: https://bugs.gentoo.org/show_bug.cgi?id=523152