Move some virtual memory constants to the top of the file where they are on
other architectures [1].

While here:
 - Remove an unused and commented out include.
 - Add a comment describing the file that other copies have.
 - Fix the style of the defines and add a comment on what each one is.

Suggested by:	[1] alc
Fix nandfs support by providing the same crc32 function as is used
in newfs_nandfs. In libstand we get crc32 from libz. The polynomial
is not the same as used for nandfs, which is the crc32 used in the
kernel.
Fix warnings (control reaches end of non-void function).
Make this WARNS=9 clean on i386 w/ clang.