Avoid some missed updates when backtracking.

This avoids unnecessarily missed updates like the following:

WARNING: One or more updates have been skipped due to a dependency conflict:

dev-util/boost-build:0

  (dev-util/boost-build-1.53.0::gentoo, ebuild scheduled for merge) conflicts with
    =dev-util/boost-build-1.52.0-r1 required by (dev-libs/boost-1.52.0-r6::gentoo, installed)

!!! The following update(s) have been skipped due to unsatisfied dependencies
!!! triggered by backtracking:

dev-libs/boost:0
_slot_confict_backtrack: simplify code

The order of backtrack_data is simply based on version comparison
since commit 9b6f69e2a66c0f1d1d6545208edb3c45eacfd845.
emerge: portage.util._argparse
prepstrip: PORTAGE_PYTHONPATH for xattr-helper.py
_argparse: support add_argument_group
xattr-helper.py: portage.util._argparse
emaint: remove callback usage

This will simplify conversion to argparse.
emaint: fix exclusive action handling

It broke in commit 42b0a6e898172c375d4d47c3346b25dac2333bf0.
emaint: portage.util._argparse
repoman: simplify help output

This will simplify conversion to argparse.
repoman: portage.util._argparse
_argparse: add ArgumentParser.error()
egencache: portage.util._argparse
emirrordist: portage.util._argparse
portageq: portage.util._argparse
chpathtool.py: portage.util._argparse
binhost-snapshot: portage.util._argparse
ebuild: portage.util._argparse
filter-bash-environment.py: don't use optparse
glsa-check: portage.util._argparse