On some versions of the server, this returns statistics concerning the server
consumption of system memory. The result is a list of lists, each in the
following format:
{<block-size>, <nused>, <nfree>}
where <block-size> is the size in bytes of a particular class of memory
fragments, <nused> is the number of such fragments currently in use in the
server, and <nfree> is the number of such fragments that have been reserved
for use but are currently free.
On servers for which such statistics are not available, `memory_usage()'
returns `{}'. The compilation option `USE_SYSTEM_MALLOC' controls
whether or not statistics are available; if the option is provided, statistics
are not available.