Fix rehash mechanism for versions of bash that complain about clobbering /dev/null

Enforce absolute RBENV_DIR to avoid having to unset CDPATH

rbenv-version-file: ensure that the version file is a file

rbenv init -: fix output to work without args and set -u

rbenv shell: better error message when shell integration wasn't enabled

Enable freezing rbenv version via rbenv version-name > .ruby-version in the shell

rbenv-which: avoid changing PATH unless necessary

rbenv-prefix: do not silence rbenv-which errors for system version