r19164: merge the gdb_backtrace script from samba4

r19164: merge the gdb_backtrace script from samba4

this more portable and try to make use of ladebug on Tru64,
but that only works when the binary is passed as 2nd arg to gdb_backtrace
as Tru64 doesn't know /proc/${PID}/exe

we need to find a way to pass the progname in 'panic action'
in samba3

metze