lib/pty-session: make wait_child callback optional

lib/pty-session: make wait_child callback optional

Now the code is duplicate on many places, but all we usually need is to
remember child status. It seems good enough to have very simple
callback child_die() to inform application about a change.

The patch also add PID to all signal related callbacks.

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