Certain values of 'wildignore', .e.g `*/node_modules/**`, would make the
provider checks not find the right executables.

Fixes https://github.com/neovim/neovim/issues/14388

Problem: ruby#Detect() and node#Detect() don't return a [prog, err] pair
which means callers must special-case them.

Solution: align their return signatures with the perl/pythonx providers.

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.

Before this commit, if user does this:
    let g:node_host_prog = '~/.nvm/versions/node/v11.3.0/bin/neovim-node-host'
the "~/" is not expanded to user's home directory.

`:help g:ruby_host_prog` suggests a path with "~/" so technically we
already claimed to support this.

closes https://github.com/neovim/node-client/issues/102

When rbenv is used for managing Ruby installations, just checking for an
executable called "neovim-ruby-host" is not enough. It has to be run as well.

If it does not return 0, then neovim-ruby-host is merely a shim for another Ruby
installation.

  $ rbenv versions
    2.5.0
  * 2.5.1 (set by /Users/mhi/.rbenv/version)

  $ rbenv whence neovim-ruby-host
  2.5.0

  $ which neovim-ruby-host
  /Users/mhi/.rbenv/shims/neovim-ruby-host

  $ neovim-ruby-host
  rbenv: neovim-ruby-host: command not found

  The `neovim-ruby-host' command exists in these Ruby versions:
    2.5.0

  $ echo $?
  127

Additionally, the detection logic was moved from provider#ruby#Detect() to
s:detect(), because the former is run in the sandbox which forbids calling
system().