env_update: always respect makelinks=False

env_update: always respect makelinks=False

This ensures that the env-update --no-ldconfig option is always
respected. Also, in order to preserve behavior during downgrades for
an env_update call in vartree.py, remove the makelinks=False for
downgrades since it wasn't respected anyway when lib path mtimes had
changed, and we don't want it to behave differently in this case now
that makelinks=False is always respected. Note that the unconditional
use of the ldconfig -X option (from bug #373341) since commit
e1347f9c0dd5ef5ff1a50d6b136b0648efb8a6ca may also come into play for
downgrades.