Merged from trunk 4883:4889

Merged from trunk 4883:4889

Rely on PYTHONPATH instead of PORTAGE_PYM_PATH for locating portage's
python modules, then fall back to an explicit sys.path insertion if an
ImportError occurs.

Fix a KeyError when the profile doesn't define default USE.

Add support for frozen config files for which updates are automatically
zapped.  Thanks to Bo ?\195?\152rsted Andresen for the initial patch
from bug #151685.

Recommend people use `emake` rather than `make` when installing with
DESTDIR.

svn path=/main/branches/prefix/; revision=4892