Adds eshutdown(2) to allow programs to stop event processing before exiting.

There is currently no way for programs using event(2) to stop event processing.
Device files and memory are held by the library.  This prevents programs using
event(2) from returning to console mode cleanly without exiting.

This patch adds eshutdown(2) to provide that function.  Existing programs using
event(2) should not be affected by this change.  I have recompiled and run
clock(1) and tweak(1) for testing, both of which behave unchanged.