logind: let {session,user,seat}_get_idle_hint() return bool and always set output...

logind: let {session,user,seat}_get_idle_hint() return bool and always set output timestamp

These helpers returned int but in practice only ever produced 0/1.
session_get_idle_hint() silently swallowed negative returns from
get_tty_atime()/get_process_ctty_atime() and fell through to
return false; user_get_idle_hint() and seat_get_idle_hint() propagated
errors from session_get_idle_hint() that could no longer occur.

Change all three to return bool and always set *t on non-NULL output.
session_get_idle_hint() previously did not set *t on the
!SESSION_CLASS_CAN_IDLE() early return; fix by defaulting *t at the
top of the function. Callers that read *t after a false return now see
DUAL_TIMESTAMP_NULL rather than uninitialized memory.

Update all callers: drop dead < 0 error checks, drop > 0 coercion,
drop DUAL_TIMESTAMP_NULL pre-init at sites that only pass &t to the
helpers.