Adding gdb_backtrace() function.
GCC warned about ignoring the return code for the system call, so
let's not ignore it.
Reorganize helper functions into anonymous namespace.

Even if your compiler doesn't support the C++ ABI demangling stuff or
GLIBC backtrace(), we can try to get a gdb backtrace.
Merge pull request #344 from libMesh/gdb_backtrace

GDB backtrace