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

Fixes #9213

ref https://github.com/neovim/neovim/issues/9001#issuecomment-434802234