Initial tracing functionality

First attempt at a low-overhead tracing feature.  To enable, you
build mtrace-ctl.c into a .so and LD_PRELOAD it.  That uses a
private API to set up a trace buffer, and calls to malloc et all
fill in records in the trace buffer.  At program exit, mtrace-ctl
reads the buffer and stores the data on disk.

Internally, the only contention point is the atomic update of the
buffer head pointer.  Once aquired, each thread fills in its
record without needing locks.

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