Prevent more variable clobbering

Was looking in lib, found a couple more places where variables were getting clobbered.

This patch prevents `$PAGER`, `$LESS`, `$LSCOLORS` & `$GREP_OPTIONS` from getting clobbered if the user had already set them.
It also quotes the path to nvm.sh in `lib/nvm.zsh` in case the path to the user's home directory has a space in it.