This patch changes the duration_* API in several ways.  First, it
updates the API to use better names.  Second, string formatting has
been removed from the API (with its associated malloc).  Finally, a
new function added to convert the time into seconds, which can be
used (or formatted) by the caller.  This eliminates hidden calls to
malloc that require associated calls to free().

This patch also removes the useless extern keyword from prototypes,
and it eliminates the duration_t typedef (use 'struct duration').
These API also allows proper error checking, as it is possible for
gettimeofday to fail in certain circumstances.

The consumers have all been chased to use this new API as well, as
there were relatively few cases doing this type of measurement.
In most cases, the code performs additional checks for errors, but
the calling code looks much cleaner in every case.