extend printf(9) %b format to allow 0 bits set

[Sean Bruno]

A small patch from my wanderings today for your review.  This patch to
kvprintf() allows me to set a %b format string of:

 "\20\0unset\1ONESET\2TWOSET"

In the case that the variable being compared has the value of 0, it will
display "0x0<unset>"

http://people.freebsd.org/~sbruno/subr_prf.txt

Sean
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