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.

`wl-copy` by default tries to determine the mime type of a copied bit of
text. From the [readme](https://github.com/bugaevc/wl-clipboard):

> wl-copy automatically infers the type of the copied content by running
> xdg-mime(1) on it.

So copying a Ruby script from Nvim may store it in the Wayland clipboard
as mime-type `application/x-ruby`.

This is a small reproduction without Nvim:

    $ cat test.rb
    #!/usr/bin/env ruby
    puts 'hello world'
    $ cat test.rb | wl-copy
    $ wl-paste --list-types
    application/x-ruby

This commit fixes that by telling wl-copy that all text copied from
Nvim has the mime type `text/plain`.

    $ cat test.rb | wl-copy --type text/plain
    $ wl-paste --list-types
    text/plain;charset=utf-8

Avoids ~30-60 ms startup cost for users of clipboard=unnamed.

Reference: https://github.com/neovim/neovim/wiki/Following-HEAD#20181118