os_open: impl mch_open with libuv. ref #133

- use return value instead of open_req.result
- libuv uv_fs_open() returns `-errno` instead of always -1
- libuv always sets open_req.result to the return value, _except_ for OOM
  where it only sets the return value. So always use the return value.
- replace calls to mch_open macro.
- update call sites expecting -1 error

src/nvim/ex_cmds.c

@@ -1575,19 +1575,19 @@ void write_viminfo(char_u *file, int forceit)
     if (tempname != NULL) {
       int fd;
 
-      /* Use mch_open() to be able to use O_NOFOLLOW and set file
+      /* Use os_open() to be able to use O_NOFOLLOW and set file
        * protection:
        * Unix: same as original file, but strip s-bit.  Reset umask to
        * avoid it getting in the way.
        * Others: r&w for user only. */
 # ifdef UNIX
       umask_save = umask(0);
-      fd = mch_open((char *)tempname,
+      fd = os_open((char *)tempname,
           O_CREAT|O_EXCL|O_WRONLY|O_NOFOLLOW,
           (int)((old_info.stat.st_mode & 0777) | 0600));
       (void)umask(umask_save);
 # else
-      fd = mch_open((char *)tempname,
+      fd = os_open((char *)tempname,
           O_CREAT|O_EXCL|O_WRONLY|O_NOFOLLOW, 0600);
 # endif
       if (fd < 0)