Bug 4300: Fix display of textual holdings (866)

Bug 4300: Fix display of textual holdings (866)

Fixed problems:
- only $z, but not $a was shown
- only first $z for each 866 field was shown
- each 866 field produced a ; even if no $z was present

After patch was applied:
- OPAC displays 866 $a and $z, separating each field with ;
- Staff displays 866 $a, $x and $z, separating each field with ;

Example(s) for testing:
866 $a1-86 (1941-1987)$xbound in 2 v. per year$zSome issues missing
866 $a1-86 (1941-1987)
866 $a--86 (1941-1897)$xinternal note
866 $zfirst note,$zsecond note

Signed-off-by: Jared Camins-Esakov <jcamins@cpbibliography.com>
Signed-off-by: Paul Poulain <paul.poulain@biblibre.com>