[RFC] impl mch_open with libuv

ref #133

update: leaving `mch_fopen` and `fdopen` (pointed out by @aktau below) for another PR.
In style guide's example in param description is indented and descriptions start with capital letters. In this case it would look like this:
```c
/// @param mode Permissions for the newly-created file (IGNORED if 'flags' is
///        not `O_CREAT` or `O_TMPFILE`), subject to the current umask
```