Small debuggerd improvements.

Include the ABI in seccomp causes.

Slightly improved command-line usage information.

Fix crasher for seccomp failures.

Bug: N/A
Test: crasher
Change-Id: Ie419ecfe72ee4f5ccf49c927be18350a58a66a90
diff --git a/debuggerd/debuggerd.cpp b/debuggerd/debuggerd.cpp
index 0c5d3cf..492e9f0 100644
--- a/debuggerd/debuggerd.cpp
+++ b/debuggerd/debuggerd.cpp
@@ -34,6 +34,8 @@
 
 static void usage(int exit_code) {
   fprintf(stderr, "usage: debuggerd [-b] PID\n");
+  fprintf(stderr, "\n");
+  fprintf(stderr, "-b, --backtrace    just a backtrace rather than a full tombstone\n");
   _exit(exit_code);
 }
 
@@ -56,7 +58,8 @@
 int main(int argc, char* argv[]) {
   if (argc <= 1) usage(0);
   if (argc > 3) usage(1);
-  if (argc == 3 && strcmp(argv[1], "-b") != 0) usage(1);
+  if (argc == 3 && strcmp(argv[1], "-b") != 0 && strcmp(argv[1], "--backtrace") != 0) usage(1);
+  bool backtrace_only = argc == 3;
 
   pid_t pid;
   if (!android::base::ParseInt(argv[argc - 1], &pid, 1, std::numeric_limits<pid_t>::max())) {
@@ -69,9 +72,8 @@
   }
 
   std::thread redirect_thread = spawn_redirect_thread(std::move(piperead));
-  bool backtrace = argc == 3;
   if (!debuggerd_trigger_dump(pid, std::move(pipewrite),
-                              backtrace ? kDebuggerdBacktrace : kDebuggerdTombstone, 0)) {
+                              backtrace_only ? kDebuggerdBacktrace : kDebuggerdTombstone, 0)) {
     redirect_thread.join();
     errx(1, "failed to dump process %d", pid);
   }