Move all usNIC stats to _stats.c|h and export them as MPI_T pvars.

This commit moves all the module stats into their own struct so that
the stats only need to appear as a single line in the module_t
definition, and then moves all the logic for reporting the stats into
btl_usnic_stats.c|h.

Further, the stats are now exported as MPI_T_BIND_NO_OBJECT entities
(i.e., not bound to any particular MPI handle), and are marked as
READONLY and CONTINUOUS.  They currently all default to verbose level
5 ("Application tuner / detailed", according to
https://svn.open-mpi.org/trac/ompi/wiki/MCAParamLevels).

Most of the statistics are counters, but a small number are high
watermark values.  Due to how counters are reported via MPI_T, none of
the counters are exported through MPI_T if the MCA param
btl_usnic_stats_relative=1 (i.e., the module resets the stats back to
zero at a given frequency).

When MPI_T_pvar_handle_alloc() is invoked on any of these pvars, it
will return a count that is equal to the number of active usnic BTL
modules.  The values returned for any given pvar (e.g.,
num_total_sends) are an array containing one value for each active
usnic BTL module.  The ordering of values in the array is both
consistent across all usnic pvars and stable throughout a single job:
array slot 0 corresponds to module X, array slot 1 corresponds to
module Y, etc.

Mapping which array slot corresponds to which underlying Linux usnic_X
device works as follows:

 * The btl_usnic_devices MPI_T state pvar is associated with a
   btl_usnic_device MPI_T enum, and be obtained via
   MPI_T_pvar_get_info().
 * If all usNIC pvars are of length N, the values [0,N) in the
   btl_usnic_device enum are associated with strings of the
   corresponding underlying Linux device.

For exampe, to look up which Linux device is reported in all usNIC
pvars' array slot 1, look up the int value 1 in the btl_usnic_devices
enum.  Its corresponding string value is underlying Linux device name
(e.g., "usnic_1").

cmr=v1.7.4:subject="usnic BTL MPI_T pvars"