In libexec, split the memory allocation method into cleared and
non-cleared. Cleared gives zeroed memory, non-cleared gives 'junk'
memory (that will be overwritten anyway, and so needn't be cleared)
that is faster to get.
Also introduce the 'memmap' method that can be used, if available,
to map code and data from executables into a process using the
third-party mmap() mode.