netrw shouldn't be touching these in the first place.

fix #11089
ref #6892
ref #6695

Partial cherry-pick of b3686b1597ea202de464df72a88fb5c76fd1b814.

* test/functional/provider: display reason from missing_provider
* provider#node#Detect: skip / handle non-existing node executable

Update runtime files.
https://github.com/vim/vim/commit/56c860c315c517d304320e12bf7b5c1479546dae

Hot fix for 0.4
This commit should be reverted when &vts option is added.

close #10831

Fixes regression due to signal being reported with exit status.
ref #10573 939d9053bdf2f56
ref https://github.com/neovim/neovim/issues/7054#issuecomment-520282429

Value of 1 cannot be used, because users might set that in their vimrc
to _disable_ a provider, which would confuse :checkhealth and has().

Instead of deciding provider status in eval_has_provider, move the
decision to the provider Vim scripts.

Previously, provider loading worked as follows:

1. eval_has_provider() verified provider availability by searching for
   the provider#providername#Call function and cached this verificaion as a static
   variable for some providers
2. providers short-circuited on loading to prevent the definition of the
   Call function (with the exception of the node provider that did not)

This commit changes the expected interface between nvim and its
providers to facilitate provider reloading, by splitting the
verification of the provider from the availability of the Call function.

eval_has_provider() now checks for a provider#providername#enabled
variable. It is up to the provider script to set this to 0 or 1
accordingly. eval_call_provider() remains unchanged.

All providers hosting a Call function were updated to respect this.

The clipboard provider now has a Reload function to reload the
provider.

Update runtime files
https://github.com/vim/vim/commit/85850f3a5ef9f5a9d22e908ef263de8faa265a95