Fix _dt_sigsegv_handler when PTRACE restrictions are active (yama)

Starting with Ubuntu 10.10, PTRACE is limited to only direct descendants.

It is controlled by /proc/sys/kernel/yama/ptrace_scope
* if it is 0, then it is not needed
* if it is 1, then this IS needed
* else there is nothing we can do (no bt will be made)

FIXME: do we need to make child wait until prctl returns in main process?

Based on:
https://bugs.kde.org/show_bug.cgi?id=245529
https://breakpad.appspot.com/166001
https://wiki.ubuntu.com/Security/Features#ptrace