grdc(6): Partial backout of r210755
grdc(6): Partial backout of r210755

The previous revision was flawed in numerous ways:
- the if condition would depend on garbage stack values
- grdc(6) would loop n times, not n seconds. This only makes
  a difference if it takes more than 1 second to recalculate/redraw
  the display.
- style(9) violations

The following change adds argument checking and tracks the elapsed time
between loops explicitly and will exit after roughly n seconds.

While here sort headers and remove bogus #ifndef

Submitted by:	bde
MFC after:	2 weeks

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