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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--) .

Suggested by Hisham.

I think this leak may still exist in the FreeBSD port.

This is what OpenBSD's top(1) does when the libkvm call fails, and it's
a good idea.

This commit also fixes process name construction. The space was being
written one character too far.

Even when they're constant, as is the case for zombie processes.

Pointed out by Michael Reed.

So that we can see errno. Pointed out by Michael Reed.

I forgot how awful the process name logic was. It was an initial hack to
get it running, and I forgot to clean it up.

I also had to change a few includes and error function uses.

err.h functions corrupts the terminal when using curses.