emerge --info: KeyError with ROOT, bug #475778

The KeyError was triggered when calling vardbapi.aux_get() on a
vardbapi instance from the wrong ROOT.
Disable global spawned_pids (avoid memory leak)

It used to be necessary for API consumers to remove pids from the
global spawned_pids, since otherwise it would accumulate a pids
endlessly. Now, spawned_pids is just an empty dummy list, so for
backward compatibility, ignore ValueError for removal on non-existent
items.
doebuild: cleanup returnpid support, bug #475812

The returnpid parameter is no longer deprecated, since it's useful for
API consumers that wish to use the fd_pipes parameter (the fd_pipes
parameter is only respected when returnpid is True).
man/portage.5: clarify package.provided virtuals
depgraph: tweak slot-operator merge order

This handles circular DEPEND/RDEPEND with one := operator, so that when
both deps are already satisfied by installed packages, the := dep is
given higher priority in merge order.
_ignore_satisfied_buildtime: tweak optional
doebuild: deprecate returnpid

This case is not supported, since it bypasses the EbuildPhase class
which implements important functionality (including post phase hooks
and IPC for things like best/has_version and die).
ForkProcess: flush stderr for traceback
doebuild: fix fd_pipes support, bug #475812

The fd_pipes was previously unsupported with returnpid=False. Now it's
fixed to pass fd_pipes down to the appropriate places.
DoebuildFdPipesTestCase: fix docstring
MergeProcess: flush stderr for traceback
Use sys.__std* streams for fileno().
calc_depclean: use runtime_slot_op priorities
doebuild: handle tmpdir / returnpid interaction
doebuild: support 'nofetch' phase
test_doebuild_fd_pipes: test pkg_info
test_doebuild_fd_pipes: test qmerge