script: use lib/pty-session

script: use lib/pty-session

This patch consolidate pseudo-terminal stuff in util-linux. From now
there is only one implementation used in su(1) --pty, scriptlive(1)
and script(1).

The new stuff is based on the original script(1) -- it means poll()
and signalfd() based.

Note that script(1) code does not provide fallback for systems/libc
where is no openpty().

Signed-off-by: Karel Zak <kzak@redhat.com>