properly support the equals slot operator in depset strings

Now we add the slot and subslot of the best installed version to depset
atoms using the equals slot operator.
remove extra newlines
drop class property from __slots__ (python3.3 fix)
fix tests failing due to testing the order of unordered types

Python 3.3 enables hash randomization by default causing tests that rely
on ordering of dicts and sets to sporadically fail.
properly clean up tmpdir after the pkg_pretend phase
update docstrings
fix grammar in REQUIRED_USE failure message
fix spelling error: nonexistant -> nonexistent
apply profile make.defaults USE settings before profile package.use

This fixes cases where the negative was applied before the positive so
the use flag was enabled, e.g. the cxx use flag for openldap which is
enabled in make.defaults of the base profile and disabled for
openldap in package.use of the base profile.
move USE and SLOT var defines to expected_ebuild_env

This mirrors portage that defines USE and SLOT for pkg_pretend which
fixes pkgs that check for enabled use flags in pkg_pretend, e.g. seabios
checks if the binary use flag is enabled in pkg_pretend.
update news
add subslot attr to atom repr
save NEEDED and NEEDED.ELF.2 files to vdb

Precursor to preserved-libs support similar to portage.
simplify snakeoil.osutils.pjoin imports
remove unused snakeoil.descriptors module
import required functions instead of entire snakeoil modules
fix plugin test for python-3.3

Previously both LowPlug and DisabledPlug classes had priorities of zero
so all three attributes used for sorting were the same for both allowing
pythoni3.3 to order them differently. Giving LowPlug a priority of one
forces its sorted position to not be interchangeable with DisabledPlug.
use the same order of identity equivalence checking everywhere
pclone_cache: explicitly define arg priorities so they get parsed in order

Without explicit priorities, all args get a default priority of 20 which
allows the target argument to be parsed before the source argument
causing the second test in the related test suite for pclone_cache to
fail.
fix line wrapping and related indentation