o reduce the overhead of calling ppsratecheck by using ticks instead of   calling getmicrouptime (but maintain the struct timeval-based calling   convention for compatibility) o eliminate the use of timersub in ratecheck
o reduce the overhead of calling ppsratecheck by using ticks instead of
  calling getmicrouptime (but maintain the struct timeval-based calling
  convention for compatibility)
o eliminate the use of timersub in ratecheck

Note that flood ping tests indicate ppsratecheck is inaccurate (but on the
conservative side) with this revised implementation.  If more accuracy is
needed we'll have to introduce an alternate interface or increase the
overhead.

Reviewed by:	silby, dillon, bde

jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase,jrobhoward/SCADAbase