terminal: make a terminal FDs non-blocking

terminal: make a terminal FDs non-blocking

Let's prevent freezes on read(2) by making a terminal FDs non-blocking.

It was discovered that there is an issue with io_uring mainloop when
multishot poll (IORING_POLL_ADD_MULTI) mode is enabled. Sometimes
false-positive poll events are put into a CQ. It makes further read(2)
stuck forever and blocks all mainloop processing for an infinite time.

Signed-off-by: Alexander Mikhalitsyn <aleksandr.mikhalitsyn@canonical.com>