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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--) .

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

- currently implemented for darwin and linux

err.h functions corrupts the terminal when using curses.

Closes #293.

https://developer.apple.com/library/mac/documentation/Darwin/Reference/ManPages/man3/signal.3.html

Change more fprintf(stderr, ...); exit(...); to err[x](...). Tweak a few existing ones and fix some style.

* size_t nmemb (number of elements) first, then size_t size
* do not assume char is size 1 but use sizeof()
* allocate for char, not pointer to char (found by Michael McConville,
  fixes #261)

Closes #228.