[RFC] Linting: Integrate in CMake, enable by default.

Similar to #1488, only for clint. There's a pretty long list of files in `RunLint.cmake` that doesn't really belong there (should be passed by caller, I think), so `[WIP]` for now.

This currently shows are a few lint issues with files in `src/nvim/os`:

```
(multiple files) #ifndef header guard has wrong style, please use: NVIM_OS_UNIX_DEFS_H
/home/florian/Projects/neovim/src/nvim/os/users.c:55:  Consider using getpwuid_r(...) instead of getpwuid(...) for improved thread safety.
/home/florian/Projects/neovim/src/nvim/os/fs_defs.h:17:  Missing space before {
```
Ha, this fix is fine IMO, since os_unix is going away. On the other hand I guess this is why people like `#pragma`. I'm not sure there is any way to prevent this issue, except to adopt a more elaborate convention such as "two underscores for a path separator":

    NVIM__OS_UNIX_DEFS_H

Or we could simply agree to avoid underscores in module names:

    `os/unixdefs.h`