Update License field to SPDX

- Additionally, the license is corrected; it should have been “BSD”
  rather than “BSD with advertising” under the old system.

arpwatch.spec

@@ -4,7 +4,16 @@ Version:        3.3
 Release:        %autorelease
 Summary:        Network monitoring tools for tracking IP addresses on a network
 
-License:        BSD with advertising
+# SPDX matching with BSD-3-Clause confirmed at
+# https://gitlab.com/fedora/legal/fedora-license-data/-/issues/49
+#
+# Any files under different licenses are part of the build system and do not
+# contribute to the license of the binary RPM:
+#   - config.guess and config.sub are GPL-3.0-or-later
+#   - configure is FSFULL
+#   - install-sh is X11
+#   - mkdep is ??? (NTP-like)
+License:        BSD-3-Clause
 URL:            https://ee.lbl.gov/
 
 Requires(pre):  shadow-utils