Revive vim_fname (-> file_exists) and fix misuse of mch_getperm.

* Move vim_fname from misc1 to os/fs:file_exists.
* Add unit tests for file_exists.
* Replace misuse of mch_getperm with file_exists.
Replace chmod with uv_fs_chmod call.
Rename file_exists to os_file_exists.
Revert to mch_getperm in readfile.