Randolph, this is in response to your bug report.
Daniel, see if this looks OK to you.  Instead of calling
pop_target (which, I agree, was iffy), we just go thru
target_mourn_inferior like we always did.

Rearranged linux_fork_killall because of an unexpected
interaction with delete_fork -- at least one fork wasn't
getting killed, because delete_fork tries to be too clever.