Retire spl_module_init()/spl_module_fini()

Retire spl_module_init()/spl_module_fini()

In the original implementation of the SPL wrappers were provided
for module initialization and cleanup.  This was done to abstract
away any compatibility code which might be needed for the SPL.

As it turned out the only significant compatibility issue was that
the default pwd during module load differed under Illumos and Linux.
Since this is such as minor thing and the wrappers complicate the
code they are being retired.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #2985