MFH: Fix race between next_in_queue and balance_pool

  mv: rename /poudriere/data/build/91amd64-pkgng/ref/poudriere/pool/unbalanced/p5-Role-Tiny-1.003.002 to /poudriere/data/build/91amd64-pkgng/ref/poudriere/building/p5-Role-Tiny-1.003.002

Reported by:	mat
MFH: Fix race between queue_empty and balance_pool.

This may have been resulting in stuck queue crashes.

Reported by:	many
MFH: Fix usage of cpdup -X/-x.

-X only accepts 1 file containing a list of files/dirs to ignore.
Use -x with .cpignore in each subdir to properly only exclude
/usr/src and /poudriere.

Note that this does not change any behavior as it worked by
accident before. /usr/src was ignore since it was null-mounted
and /poudriere was ignored since it was listed 2nd as -X
which is ignored itself.

Reported by:	John Hein
MFH: bulk: Avoid doing a rollbackfs on first builds on a fresh mount