LP#1464709: Disentangle hold bits from is_available

LP#1464709: Disentangle hold bits from is_available

We need to avoid conflating a status's available equivalence with its
desirablity as a hold target.  So, we will leave the "On holds shelf"
status as special in circulation, and not touch the hold targeter or
pull list code.  That's the primary effect of this commit.

Instead, a future revision will add something like the is_available
flag, but defining the targetability of a status, and use that in the
targetter.  Further use of is_available may happen once is_targetable (or
whatever it ends up being called) is created.

Signed-off-by: Mike Rylander <mrylander@gmail.com>