The use case is different in vcl and xmlsecurity: vcl creates a new PDF
(possibly with a signature), while xmlsecurity signs an existing PDF,
but this part can be shared between the two.

So far in vcl only the nss part is moved, not touching mscrypto yet.

Change-Id: Ie776f622c1a4a3a18e79e78f68722a2fa219a83b
Reviewed-on: https://gerrit.libreoffice.org/30063Reviewed-by: Miklos Vajna <vmiklos@collabora.co.uk>
Tested-by: Jenkins <ci@libreoffice.org>