feat(fd): implement fcntl(F_GETFL)

[mkroening]

This implements fcntl(F_GETFL) in three steps:

1. Move file status flags (O_APPEND and O_NONBLOCK) from OpenOptions into a new fd::StatusFlags type as per POSIX.
2. Replace ObjectInterface::ioctl(&self, cmd: IoCtl, value: bool) with ObjectInterface::set_status_flags(&self, status_flags: fd::StatusFlags), as that seems more straightforward for adding methods compared to adding more values to IoCtl and handling those in the implementors.
3. Implement fcntl(F_GETFL) by adding ObjectInterface::status_flags(&self).

This closes #1383.