Add DEFAULT_NOMOUSE option to disable default mouse=a

[dereks_lifeofadishwasher.com]

Summary

With commit https://github.com/vim/vim/commit/8c08b5b now has
$RUNTIME/defaults.vim. This enables mouse=a for all terminals that have
mouse runtime support.

This is problematic for terminals that can't bypass mouse support with
shift-click. Upstream is reluctant to remove this from defaults.vim.

Having mouse=a in defaults.vim also requires you to have a .vimrc for
all users where mosue=a is problematic.

Test Plan

Build with DEFAULT_NOMOUSE on/off and see mouse=a disbaled/enabled in
$RUNTIME/defaults.vim

[adamw]

I really don't want to have to have an option to set a default unless we absolutely have to. I'm not entirely clear on the effects of "set mouse=". Can THAT be the default?

[dereks_lifeofadishwasher.com]

Good point I was only thinking about vim via terminal.

set mouse= should disable mouse movements for all terminal types however it can still be used for mouse selection with the GUI. I could see this causing problems with people that use gvim without a mouse setting in ~/.vimrc however if you're setting DEAFULT_NOMOUSE you would know this.

If you were looking to set the default before 8c08b5b it was to not set mouse at all and let the runtime set it, see :h mouse (mouse=a for GUI, mouse="" for Terminal). If so then removing the whole if has('mouse') ... endif block would do that.

Could this be done at other location than defaults.vim? I don't think DEFAULT_VIMRC would work since not everyone uses that option.

Are you suggesting remove DEFAULT_NOMOUSE and to turn post-install-DEFAULT_NOMOUSE-on: to post-install:

Maybe the only possible set of users that might object to this option would be users that got use to mouse moving since 8c08b5b (less than a year ago) ~Sept/Oct 2016 for ports users.

[dereks_lifeofadishwasher.com]

Reviewing the way Debian handled this same issue maybe it's best left to the user to handle this. https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=837761