Figure out fault inheritance on FreeBSD with krfexec

[woodruffw]

We should come up with a way to have child processes (arbitrarily deep, not just the first level of children) inherit the targeted status of the original krfexec'd parent on FreeBSD.

Some ideas:

• In our fault wrapper, check whether our current PID is the child of the targeted PID
  • This is cheap for the first level of children, since FreeBSD's struct proc contains p_oppid. For deeper levels we'll probably have to walk up the tree, which would (probably) be too intensive and not very kthread/MP-safe
• LD_PRELOAD or otherwise wrap fork inside krfexec spawned programs, so that fork checks whether it's the child of a process being faulted and adds the new PID to a set of faulted PIDs
  • This requires us to update the targeting system to allow multiple PIDs, which we should do at some point anyways
• Maybe something really crazy where we wrap the dynamic linker and put some magic somewhere in the loaded program
• Maybe a special memory region + a new targeting mode that krfexec uses by default instead? Child processes should inherit things like memory maps, so this might not be too hard to do
• Similar to the dynamic linker: could we abuse the ELF auxiliary vector for this?