The exists('g:loaded_foo') convention is rather common, and may be
relied upon in some cases. It's also very unlikely that a user or plugin
has any reason to set g:loaded_foo to zero, so the principle of least
surprise can be brushed aside here.

https://github.com/neovim/neovim/issues/6107#issuecomment-279532143

Closes #6107

Fixes #5926.

Old versions of `xsel` do not support some required features.
Fall back to `xclip` then.

Closes #5853

This option simplifies the configuration options:

1) `g:python{,3}_host_prog` is not set.

    Neovim tries its best to find a suitable interpreter. This means calling
    exepath(), potentially multiple times, and a system('python -c ...') with
    the first found interpreter, to get the Python version.

2) `g:python{,3}_host_prog` is set.

    Avoids everything of the above. No safety checks, no training wheels. Fast
    host startup time!

Never throw an error when provider/clipboard.vim is sourced for the first time.
Save the error instead and expose it via `provider#clipboard#Error()`, mimicking
provider/python.vim.