sys/_bitset.h: Fix fall-out from commit 5e04571cf3c

The changes to the bitset macros allowed sched.h to be included into
userland programs without name space pollution due to BIT_* and
BITSET_* macros.

The definition of a global variable "bitset" had been overlooked.
This name space pollution caused a compile failure in print/miktex.

This commit renames the bitset variable to __bitset with the same
mapping back to the bitset if _KERNEL or _WANT_FREEBSD_BITSET is
defined.

This fix has been suggested by kib. It has been tested to let the
build of the print/miktex port succeed and to not break buildworld.

This commit shall be MFCed together with commit 5e04571cf3c.

Reported by: arrowd
MFC after: 1 month