epoch(9): cleanups, additional debug checks, and add global_epoch

• GC the _nopreempt routines
  • to really benefit we'd need a separate routine
  • they're not currently in use
  • they complicate the API for no benefit at this time

• check that we're actually in a epoch section at exit

• handle epoch_call() early in boot

• Fix copyright declaration language

Approved by: sbruno@