Add #ifdefs in the mpr(4) driver so that versions of stable/9 that
have implemented the PIM_NOSCAN rescan functionality will have it
enabled.

This is a no-op for head.

Reviewed by:	slm
Sponsored by:	Spectra Logic Corporation
MFC after:	3 days
Use KASSERTs as suggested by glebius@

MFC after: 3 days
X-MFC with: 265691
Fix two more typos.

Submitted by:	Trond Endrestol