Add experimental 'runevery' program.

runevery [-d <count dir>] <count1> <cmd1> <count2> <cmd2>

This utility runs one of cmd1 or cmd2 every time it is called. A count is kept
in a temporary file so that runevery knows when to switch between cmd1 and cmd2.
cmd1 is first run count1 times, then cmd2 is run count2 times (i.e.  count1
calls to runevery call cmd1; then count2 calls to runevery call cmd2); the
counter is then reset and the process begins anew.