public functions did not have an isc__ prefix, and
                        referred to functions that had previously been
                        renamed.

 184.   [cleanup]       Variables/functions which began with two leading
                        underscores were made to conform to the ANSI/ISO

function declaration and comment reformatting in accordance with coding
style document.

check_overrun conditionally compiled based on ISC_MEM_FILL and
ISC_MEM_CHECKOVERRUN to avoid compiler warnings about being defined
but not used if one of those two CPP symbols is not defined.