mtr: extend gdb backtace info

bt full - to include args and locals.

set print sevenbit on
  - it is more useful to be able to see the exact bytes
    (in case something is dumped as a string and not hexadecimal digits)
set print static-members off
  - there are many interesting (non-const) static members
set frame-arguments all
  - even non-printables are useful to see.

Let's make our bb logs give a little bit more detail on those
hard to reproduce bugs.

Tests on rhel7's gdb-7.6.1-120.el7

mysql-test/lib/My/CoreDump.pm

@@ -78,7 +78,10 @@ sub _gdb {
   my ($tmp, $tmp_name) = tempfile();
   print $tmp
     "bt\n",
-    "thread apply all bt\n",
+    "set print sevenbit on\n",
+    "set print static-members off\n",
+    "set print frame-arguments all\n",
+    "thread apply all bt full\n",
     "quit\n";
   close $tmp or die "Error closing $tmp_name: $!";