LP#1539088: marc_export prints "Waiting for input" when non-interactive

LP#1539088: marc_export prints "Waiting for input" when non-interactive

This commit modifies marc_export to only print "Waiting for input"
when run in an interactive manner.  This suppresses the output when
run from a pipe.

To test this change:

Before the commit is applied both of the following command lines will
output "Waiting for input" to standard error:

$ marc_export --format XML

$ echo <bibid> | marc_export --format XML

Where you replace <bibid> with a bibliographic record entry id from
your database.

After the commit is applied, only the first command line will output
the "Waiting for input" message.

Signed-off-by: Jason Stephenson <jstephenson@mvlc.org>
Signed-off-by: Ben Shum <ben@evergreener.net>