A set of routines are provided to enable a task to send messages to itself. ams_kick() is intended
to enable main-line code to queue a message which it can subsequently collect by using
ams_receive().
The other functions are intended only for calling from within signal handlers. ams_resmsg(),
ams_astmsg() and ams_astint() generate messages to be read by ams_receive() but ignored by
ams_getreply().
ams_extint() generates a message which can be received by either ams_receive() or ams_getreply()
(its main purpose is to allow user interfaces to be implemented).
At the receiving end, these messages result in the appropriate value of message_status, one
of: