...which are now only ever included from C++ code.  Drop extern "C" where
appropriate (i.e., generally everywhere) and fix the occasional ensuing loplugin
warning.  (But no further C++'ification for now.)

Change-Id: I78830692888ee0ae0cac49878042effad2d6707f
Reviewed-on: https://gerrit.libreoffice.org/34865Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>