gnu: Move nss & co. to nss.scm.

gnu: Move nss & co. to nss.scm.

* gnu/packages/gnuzilla.scm (nspr, nss): Move to...
* gnu/packages/nss.scm: ... here.  New file.
* gnu/packages/chromium.scm, gnu/packages/disk.scm,
gnu/packages/freedesktop.scm, gnu/packages/gnome.scm,
gnu/packages/gnunet.scm, gnu/packages/java.scm,
gnu/packages/libreoffice.scm, gnu/packages/linux.scm,
gnu/packages/mate.scm, gnu/packages/openldap.scm,
gnu/packages/package-management.scm, gnu/packages/password-utils.scm,
gnu/packages/polkit.scm, gnu/packages/qt.scm,
gnu/packages/sssd.scm, gnu/packages/storage.scm,
gnu/packages/vpn.scm, gnu/packages/web.scm, gnu/packages/xml.scm: Adjust
accordingly.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add nss.scm.