Added SPDX License ID to all EPICS-original source files

In some cases the license-identification header was missing,
so I added that as well. Replaced the remaining headers that
specifically identified "Versions 3.13.7 and higher".

Makefiles and the build system were deliberately excluded.

modules/libcom/test/epicsThreadPriorityTest.cpp

@@ -3,6 +3,7 @@
 *     National Laboratory.
 * Copyright (c) 2002 The Regents of the University of California, as
 *     Operator of Los Alamos National Laboratory.
+* SPDX-License-Identifier: EPICS
 * EPICS BASE is distributed subject to a Software License Agreement found
 * in file LICENSE that is included with this distribution.
 \*************************************************************************/