The neovim module is available for backwards compatibility.  We should
not yet force the use of the pynvim module, since there's no other major
reason to bump the minimum supported Python client module.

Closes #9426

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

PR #9304 added support for functions in clipboard providers. As part of
the PR I meant to move two checks in the provider code out of an if
statement into separate statements and adding additional checks for
g:clipboard attributes - as it turns out the code is wrong and it does
not implement additional checks while it adds two conditions that make
very little sense

    type(g:clipboard['copy']) #isnot# v:t_func

what would make sense would be something along the lines of

    type(g:clipboard['copy']['+']) #isnot# v:t_func

but might not be what we want either, so I'm reverting this.

Up to now g:clipboard["copy"] only supported string values invoked as
system commands.

This commit enables the use of VimL functions instead. The function
signatures are the same as in provider/clipboard.vim. A clipboard
provider is expected to store and return a list of lines (i.e. the text)
and a register type (as seen in setreg()).

cache_enabled is ignored if "copy" is provided by a VimL function.

The order was swapped in #4150 to prefer `xsel` but there wasn't a clear
explanation.  Meanwhile, `xsel` has been neglected upstream.

Let's trying preferring `xclip` again, we've had a few reports of
problems with `xsel`.

closes #7237
ref #5853
ref #7449

Win32 allows omitting the `.exe` extension, but WSL does not. 

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