[Runtime indent plugins] 3 Things about 'shiftwidth', 'softtabstop'

[tyru]

Hi, Vimmers.

I'm sending this email to Vim indent plugins maintainers and vim_dev.
I suppose all of you maintain `$VIMRUNTIME/runtime/indent/*.vim`.
I want you to review and merge my modifications.

Now I'm working on the issue about Vim indent plugins.
(See details below)
https://github.com/vim/vim/pull/578

Please see my detailed commits for above link.
In brief:
1. I've changed `&shiftwidth` to `shiftwidth()`.
2. Removed a checking code / a wrapper function for shiftwidth()
for some specific indent plugins.
3. Some indent plugins change `shiftwidth` and `softtabstop`.
Is it evil? What do you think?

1. I've changed `&shiftwidth` to `shiftwidth()`.

This allows user to `set shiftwidth=0` in .vimrc.
This feature is added in 7.3.629 (more than 3 years ago,
but it makes no sense because indent plugins do not support this yet)

2. Removed a checking code / a wrapper function for shiftwidth()
for some specific indent plugins.

I removed a checking code / a wrapper function for shiftwidth().
* bzl.vim
* go.vim
* html.vim
* teraterm.vim
* yaml.vim

3. Some indent plugins change `shiftwidth` and `softtabstop`.
Is it evil? What do you think?

Nowadays, Vim supports syncing `shiftwidth` and `softtabstop` values
with `tabstop`, like this.

set tabstop=2 shiftwidth=0 softtabstop=-1

shiftwidth: 7.3.629 or later
softtabstop: 7.3.692 or later

This makes it easy to change indent behavior at once.
A user can change it only changing `tabstop` value.
But, some indent plugins seem to change `shiftwidth` or `softtabstop` values.
As a result, a user must set all 3 options by him or herself.


I'm waiting for your feedbacks about my modification.

Thanks