Mark std.stdio.File.this, eof and other methods as @safe or @trusted

This pull requst marks `std.stdio.File.this`, `eof`, `tell`, `setvbuf`, and `fileno` as `@trusted` and
`clearerr` and `rewind` as `@safe`.

- `File.this` uses system function `core.stdc.stdlib.malloc` and unsafe cast but we can verify that it can be tursted.
- `eof`, `tell` and `fileno` remove `const` from `_p.handle` but they do not change `_p.handle`.
- `setvbuf` uses system function `core.stdc.stdio.setvbuf` but we can verify that it can be trusted.