eventq: new eventq data structure. The new data structure is singly

eventq: new eventq data structure.  The new data structure is singly
linked list sorted by time and priority.  For things of the same time
and priority, a second, circularly linked list maintains the data
structure.  Events of the same time and priority are now inserted in
FIFO order instead of LIFO order.  This dramatically improves the
performance of systems that schedule multiple events at the same time.

The FIFO order version is not preferred to LIFO (because it may cause
people to rely on it), but I'm going to commit it anyway and
immediately commit the preferred LIFO version on top.