Mostly, several functions that take pointers as arguments, almost
always char * pointers, had those pointers qualified with "const".
Those that returned pointers to previously const-qualified arguments
had their return values qualified as const.  Some structure members
were qualified as const to retain that attribute from the variables
from which they were assigned.

The macro DE_CONST was added to isc/util.h to deal with a handful of very
special places where something is qualified as const but really needs to have
its const qualifier removed.

Also cleaned up a few places where variable names clashed with reserved
identifiers.  (Which mostly works fine, but strictly speaking is undefined
by the standard.)

Minor other ISC style cleanups.