Allow disabling passing of cert file

src/imapdomo.skel

             "-l          list handlers and exit"                              \
             "-d          turn on debugging mode"                              \
             "-t CERTS    Use certificates file CERTS."                        \
+            "-T          Use system CA certificate storage."                  \
             "-N          turn off locking; make sure that handlers cannot"    \
             "            interfere with each other or that another locking"   \
             "            mechanism is in place.  Also see NOTE below."        \
         -t) Certs="$2"; shift 2 || usage -w "missing value to: $1" ;;
         -l) lshandlers; exit ;;
         -N) NoLock=true; shift ;;
+        -T) Certs=; shift ;;
         -V|--version-semver) show_semversion ;;
         --version) show_version ;;
         -*) usage -w "unknown argument: '$1'" ;;