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/nonEpicsThreadPriorityTest.cpp

@@ -1,3 +1,9 @@
+/*************************************************************************\
+* SPDX-License-Identifier: EPICS
+* EPICS BASE is distributed subject to a Software License Agreement found
+* in file LICENSE that is included with this distribution.
+\*************************************************************************/
+
 #include <epicsThread.h>
 #include "epicsUnitTest.h"
 #include "testMain.h"