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Fix by @Explorer09 (see discussion on #355).

GRAPH_HEIGHT macro and 'dot' variable

(Cherry-picked from d56bcd8e, the
experimental graph coloring branch)

The variable 'dot' in GraphMeterMode_draw now means "maximum number of
dots per value (column) in graph". The old meaning was "amount of value
that is to be represented by a dot" and was always a fraction. Due to
a limitation in floating point computing, if GRAPH_HEIGHT were not a
power of 2, then rounding errors will occur on numbers like (1.0/3).
(Currently GRAPH_HEIGHT is 4 and so no precision loss.) 'dot' was used
as a divisor, and it's "division by a reciprocal". We change that to
simple multiplication.

(Cherry-picked from e93028d7, the
experimental graph coloring branch)

Currently GRAPH_HEIGHT=4 . This prevents hard-coding the height of the graph
meters, and allows user to change it at compile-time.

Introduce CLAMP macro. Unify all MAX(l,MIN(h,x)) uses.

With the CLAMP macro replacing the combination of MIN and MAX, we will
have at least two advantages:
1. It's more obvious semantically.
2. There are no more mixes of confusing uses like MIN(MAX(a,b),c) and
   MAX(MIN(a,b),c) and MIN(a,MAX(b,c)) appearing everywhere. We unify
   the 'clamping' with a single macro.
Note that the behavior of this CLAMP macro is different from
the combination `MAX(low,MIN(x,high))`.
* This CLAMP macro expands to two comparisons instead of three from
  MAX and MIN combination. In theory, this makes the code slightly
  smaller, in case that (low) or (high) or both are computed at
  runtime, so that compilers cannot optimize them. (The third
  comparison will matter if (low)>(high); see below.)
* CLAMP has a side effect, that if (low)>(high) it will produce weird
  results. Unlike MIN & MAX which will force either (low) or (high) to
  win. No assertion of ((low)<=(high)) is done in this macro, for now.

This CLAMP macro is implemented like described in glib
<http://developer.gnome.org/glib/stable/glib-Standard-Macros.html>
and does not handle weird uses like CLAMP(a++, low++, high--) .

Darwin: replace vm_statistics64_* with vm_statistics_*

Works with:
- Darwin 9.8.0 (OS X 10.5.8) PPC
- Darwin 15.2.0 (OS X 10.11.2) Intel

vi keys: translate ALT-h/j/k/l to arrow keys

Thank you @Explorer09 for the report!

Fix typo: prority => priority

add some security checks when running SUID root

use AC_HELP_STRING for proc dir

Rename variable for consistency

Suggested by Hisham.

align help output