terminal: Add API for flushing input buffer

terminal: Add API for flushing input buffer

In the future we're going to start reading keyboard input from
/dev/input instead of the tty. When that happens, input will
still be sent by the kernel to the tty.

This input would eventually back up and overflow.

To address that problem before it exists, this commit adds a new
API, ply_terminal_flush_input, that will drain the input buffer
on demand.

We can later use that API to keep the tty from backing up.