can't gdb across fork

[Simon 'corecode' Schubert]

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.

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