2 BUGFIX (1 being unimarc specific) : change in plugin handling

2 BUGFIX (1 being unimarc specific) : change in plugin handling

before this commit, the main (usually $a) entry could not be entered by the librarian.
With the new BiblioAddAuthority feature, it's a bad idea : the librarian will
be able to enter a value manually, and the authority will be automatically created on the fly
Thus the small change in addbiblio.pl

This commit also fixes a bug in plugin 4XX (which is unimarc specific) :
"Empty" option did not work properly + API convention fix (biblionumber instead of bibnum)

Signed-off-by: Chris Cormack <crc@liblime.com>