bmake_malloc and friends. Implement them via macros for the native case
and provide fallback implementations otherwise. Avoid polluting the
namespace by not defining enomem globally. Don't bother to provide
strdup and strndup, they were only used for the estrdup and estrndup
comapt code.
This addresses the presence of emalloc in system libraries on A/UX and
resulted strange issues as reported by Timothy E. Larson.
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