Bug 6201: Add fields 1xx to marc2bibtex (for MARC21 and NORMARC)

Bug 6201: Add fields 1xx to marc2bibtex (for MARC21 and NORMARC)

Modified Record::marc2bibtex to varlidate fields 100, 110 and 111 in
non-UNIMARC flavours.

Test plan:
1)Search any books in the OPAC with a main entry (1XX in MARC21, 700-720 in UNIMARC)
2)Export the record in the bibtex format
==>The output won't contain the main entry.
3)Apply the patch
4)Export the record
==>The record will contain the main entry

Signed-off-by: Chris Cormack <chris@bigballofwax.co.nz>
Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Fixes a long standing bug.
Passes all tests and QA script.
Tested with multiple records, seems to work well.

Signed-off-by: Katrin Fischer <Katrin.Fischer.83@web.de>
Signed-off-by: Galen Charlton <gmc@esilibrary.com>