LP#2004587: add option to marc_export to tune 852$b output

LP#2004587: add option to marc_export to tune 852$b output

The new --852b switch (when used in conjunction with --items)
takes the following values:

 * circ_lib - emit the item circulation library in 852$b
 * owning_lib - emit the owning library in 852$b
 * both - emit owning lib and circ lib as separate repeats
   of $b. This is both the default and legacy behavior.

To test
-------
[1] Set up some items with differing values for the owning
    library and the circulating library.
[2] Run several exports exercising the three --852b options
    and verify that the 852 $b output matches what is specified
    by the switch.

Signed-off-by: Galen Charlton <gmc@equinoxOLI.org>