Reintroduce timer for transport cmd channel, don't wake_up unconditionally.

Reintroduce timer for transport cmd channel, don't wake_up unconditionally.

Revert parts of commit a85c8a "runtime/io.c: Explicitly signal setting of
_stp_exit_flag" and commit 46ac9e "Remove _stp_ctl_work_timer from module
transport layer". Introduce a new test wake_up.exp that shows a deadlock
when sending cmd messages and waking up the reader immediately.

Renamed _stp_ctl_write to _stp_ctl_send, which can be called from
everywhere. Rename _stp_ctl_send to _stp_ctl_send_notify that can be
called from user context in the transport layer itself (this will
immediately notify any readers). Document all places that use
_stp_ctl_send_notify directly to clarify why that is safe.

See http://sourceware.org/ml/systemtap/2011-q3/msg00163.html