vim: Set mouse=a in system vimrc

vim: Set mouse=a in system vimrc

The mouse=a was removed after updating to vim 8 as vim by default
now sets mouse=a. But this only works if no ~/.vimrc is found - if
there is one then mouse won't be set. As we really want mouse=a on
Termux since it is that useful on touch screens we enable it again
regardless if there is a ~/.vimrc or not.

Fixes #912.