Refactor the CRC-32 code to enhance its usability. Move the actual

CRC logic to a new function: crc32_raw() that obtains the initial
CRC value as well as leaves any post-processing to the caller. As
such, it can be used when the initial CRC value is not ~0U or when
the final CRC value does need to be inverted (bitwise). It also
means that crc32_raw() can be called repeatedly when the data is
not available as a single block, such as for scatter/gather lists
and the likes.

Avoid the additional call overhead incured by the refactoring by
moving the implementation off crc32() to sys/systm.h and making it
inlinable. Since crc32_raw() is itself trivial and since it may
be used in loops that iterate over fragments, having it available
for inlining can be beneficial. Hence, move its implementation
to sys/systm.h as well.

Keep the original implementation of crc32() in libkern/crc32.c for
documentation purposes (as a comment of course).

Triggered by: Jose M Rodriguez (josemi at freebsd dot jazztel dot es)
Discussed on: current@
Tested on: amd64, ia64 (BVO having GPT partitions)
Jargon file candidate: BVO = By Virtue Of :-)

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