wmmon: Properly compute height of system load window.

wmmon: Properly compute height of system load window.

Previously, we looped through the history and added 100 whenever we found
a larger value.  This has a number of problems.  In particular,
* We get a maximum possible value of 5500 (100 * the number of values in
  the history).  It is certainly possible to have a system load north of
  this on modern systems.
* If the system load in history were to jump by more than 100 in a single
  step, then we wouldn't be adding enough.  For example, suppose the
  system load in history is 175, and our height was previously computed
  to be 200.  Suppose the next value in history is 320.  We would add
  100 to get a new height of 300, which isn't sufficient to display the
  320.

The fix is simple -- replace the if statement with a while loop, i.e.,
continue adding 100 until we get a height that fits our value.