Add kmem_asprintf(), strfree(), strdup(), and minor cleanup.

Add kmem_asprintf(), strfree(), strdup(), and minor cleanup.

This patch adds three missing Solaris functions: kmem_asprintf(), strfree(),
and strdup().  They are all implemented as a thin layer which just calls
their Linux counterparts.  As part of this an autoconf check for kvasprintf
was added because it does not appear in older kernels.  If the kernel does
not provide it then spl-generic implements it.

Additionally the dead DEBUG_KMEM_UNIMPLEMENTED code was removed to clean
things up and make the kmem.h a little more readable.