Bug 20097: Use same regex in marc2dcxml and opac-export

Bug 20097: Use same regex in marc2dcxml and opac-export

This regex allows: dc, oaidc, srwdc, and rdfdc.

Test plan:
[1] Check https://[yourserver]/cgi-bin/koha/opac-export.pl?bib=[some_biblionumber]&op=export&format=dc
[2] Check https://[yourserver]/cgi-bin/koha/opac-export.pl?bib=[some_biblionumber]&op=export&format=oaidc
[3] Check https://[yourserver]/cgi-bin/koha/opac-export.pl?bib=[some_biblionumber]&op=export&format=nodc. This one should trigger a not-supported message.

Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Mark Tompsett <mtompset@hotmail.com>
Signed-off-by: Josef Moravec <josef.moravec@gmail.com>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>