xgettext: Allow multiple --copyright-holder
Feature requested by Francesco Poli in:
<https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=682580>.
* xgettext.c (default_copyright_holder): New constant, renamed
from copyright_holder.
(copyright_holder): Define as a variable.
(main): Allow multiple --copyright-holder options.
(construct_header): Support multiple --copyright-holder options.
