Improved non-statistical (GUPROF) profiling:

Improved non-statistical (GUPROF) profiling:
- use a more accurate and more efficient method of compensating for
  overheads.  The old method counted too much time against leaf
  functions.
- normally use the Pentium timestamp counter if available.
  On Pentiums, the times are now accurate to within a couple of cpu
  clock cycles per function call in the (unlikely) event that there
  are no cache misses in or caused by the profiling code.
- optionally use an arbitrary Pentium event counter if available.
- optionally regress to using the i8254 counter.
- scaled the i8254 counter by a factor of 128.  Now the i8254 counters
  overflow slightly faster than the TSC counters for a 150MHz Pentium :-)
  (after about 16 seconds).  This is to avoid fractional overheads.

files.i386:
permon.c temporarily has to be classified as a profiling-routine
because a couple of functions in it may be called from profiling code.

options.i386:
- I586_CTR_GUPROF is currently unused (oops).
- I586_PMC_GUPROF should be something like 0x70000 to enable (but not
  use unless prof_machdep.c is changed) support for Pentium event
  counters.  7 is a control mode and the counter number 0 is somewhere
  in the 0000 bits (see perfmon.h for the encoding).

profile.h:
- added declarations.
- cleaned up separation of user mode declarations.

prof_machdep.c:
Mostly clock-select changes.  The default clock can be changed by
editing kmem.  There should be a sysctl for this.

subr_prof.c:
- added copyright.
- calibrate overheads for the new method.
- documented new method.
- fixed races and and machine dependencies in start/stop code.

mcount.c:
Use the new overhead compensation method.

gmon.h:
- changed GPROF4 counter type from unsigned to int.  Oops, this should
  be machine-dependent and/or int32_t.
- reorganized overhead counters.

Submitted by:	Pentium event counter changes mostly by wollman