csmock --limit-msg-len: limit length of diagnostic messages

... defaulting to 512 to avoid resource exhaustion on scans that
go wild.

Reproducer:
```
$ csmock -r rhel-7-x86_64 -f nss-util-3.90.0-1.el7_9.src.rpm -t gitleaks
```

Resolves: https://issues.redhat.com/browse/OSH-67
Closes: #128

py/csmock

@@ -764,6 +764,11 @@ key event (defaults to 3).")
         help="stop processing a warning if the count of its occurrences \
 exceeds the specified limit (defaults to 1024).")
 
+    parser.add_argument(
+        "--limit-msg-len", type=int, default=512,
+        help="limit length of diagnostic messages by the specified number of chars \
+(defaults to 512).")
+
     parser.add_argument(
         "-k", "--keep-going", action="store_true",
         help="continue as much as possible after an error")
@@ -908,6 +913,9 @@ exceeds the specified limit (defaults to 1024).")
     if args.warning_rate_limit > 0:
         props.results_limits_opts += [f"--warning-rate-limit={args.warning_rate_limit}"]
 
+    if args.limit_msg_len > 0:
+        props.results_limits_opts += [f"--limit-msg-len={args.limit_msg_len}"]
+
     if args.run_check:
         # run the %check section of specfile
         props.run_check = True