Relocate vim swap/backup/undo files

The location where swap/backup/undo files should be in a subdirectory of
a vim runtimepath, and one that is user specific.  The idea of using
something like the previous .local/shared/vim is too linux distro
specific.

Since MacVim and *nix vim both have $HOME/.vim in the runtimepath, then
this is the best place.  For Windows, we have to use $HOME/vimfiles
instead.