[twic]

I think they're all used for FFI:

- in ui.rs, for resetting the blocking state of stdin via fcntl

- in tuntap.rs, for an ioctl to do some tun/tap setup, and resetting errno beforehand

- in pty.rs, for ioctls for controlling terminal admin and setting the terminal size

- in util.rs, for getting the current UID, calling getpwnam or getpwuid, and extracting field values from the resulting passwd struct

I have to confess that i don't know the details of the Linux API well enough to know for sure, but they all look eminently reasonable.