fbsd-nat: Fix several issues with detaching.

- Detach from any child processes implicitly attached to by the kernel
  due to fork following that have not yet been processed by GDB's
  core.

- Delete breakpoints before detaching.

  inf-ptrace::detach does not do this (somewhat surprisingly), so add
  an override to remove breakpoints from a process before detaching
  from it.

  This also requires explicitly draining any pending SIGTRAP events
  for software breakpoints before detaching.  In particular, threads
  may need their PC adjusted due to the software breakpoint before
  being resumed after detach.  On more modern systems using the si_code
  from SIGTRAP to identify software breakpoint traps, the PC is adjusted
  in ::wait_1 as a side effect of parsing the event.  To support older
  kernels, ::detach fixes up the PC for any SIGTRAP stop whose potential
  new PC matches an existing software breakpoint.

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