Add bit_count to the bitstring(3) api

Add bit_count to the bitstring(3) api

Add a bit_count function, which efficiently counts the number of bits set in
a bitstring.

sys/sys/bitstring.h
tests/sys/sys/bitstring_test.c
share/man/man3/bitstring.3
	Add bit_alloc

sys/kern/subr_unit.c
	Use bit_count instead of a naive counting loop in check_unrhdr, used
	when INVARIANTS are enabled. The userland test runs about 6x faster
	in a generic build, or 8.5x faster when built for Nehalem, which has
	the POPCNT instruction.

sys/sys/param.h
	Bump __FreeBSD_version due to the addition of bit_alloc

UPDATING
	Add a note about the ABI incompatibility of the bitstring(3)
	changes, as suggested by lidl.

Suggested by:	gibbs
Reviewed by:	gibbs, ngie
MFC after:	9 days
X-MFC-With:	299090, 300538
Relnotes:	yes
Sponsored by:	Spectra Logic Corp
Differential Revision:	https://reviews.freebsd.org/D6255