[NTG-context] Update overwrites configuration

Hi, 1. Edit .../tex/texmf/web2c/texmfcnf.lua 2. Set TEXMFCACHE = "/var/cache/context/texmf-cache" 3. Run first-setup.sh to update ConTeXt. Expected Results TEXMFCACHE remains "/var/cache/context/texmf-cache" Actual Results TEXMFCACHE reverts to "$SELFAUTOPARENT/texmf-cache" If the values have been changed, then: (a) updates should not overwrite the new values; (b) an option to prevent overwriting new values on upgrade should be available; (c) user should be prompted to overwrite changed configuration values; (d) user should be prompted to accept overwriting each new value; or (e) a backup of the configuration file should be made, followed by a message that the changes have been overwritten and previous values archived.