can't gdb across fork
When the debugged process performs a fork(), gdb/ptrace won't notice and
will not be able to remove breakpoints in the new child. When the child
then hits a breakpoint, it will receive a SIGTRAP and dump core.
gdb needs to be aware of forks, so that it will be able to remove the
breakpoints in the child.
Test: gdb sh and break stalloc.