move vim tmp to var/tmp/vim, softlink tmp to /tmp

~/tmp will now point to /tmp so that it will be cleared regularly on
systems that have temporary /tmp storage. been meaning to do this for a
while but this is a more stable solution than anything else. explicitly
calls for BSDs to avoid issues with eg. termux

vim will now use ~/var/tmp/vim instead of ~/tmp/vim so that its files
will remain longer

adjusted install.sh for these changes + some cleanup