Add an "--all" option to marc_export and silence some warnings

Add an "--all" option to marc_export and silence some warnings

Rather than forcing users to generate a set of record IDs using a
separate process, the --all option enables to to export all records
of the specified type. Note that the approach is basic: grab the
record with the highest ID, then start at ID 0 and count up by 1
while trying to retrieve each record along the way. While this will
undoubtedly generate plenty of warnings for deleted records or gaps
in the ID sequences, the alternative of retrieving an ID list for
potentially millions of records in large Evergreen instances wasn't
appealing.

Also, $count{did} and $count{bib} resulted in uninitialized variable
warnings if no records were actually exported, so initialize them to 0.

git-svn-id: svn://svn.open-ils.org/ILS/trunk@19090 dcc99617-32d9-48b4-a31d-7c20da2025e4