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.
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