fbsd-nat: Fix resuming and waiting with multiple processes.

I did not fully understand the requirements of multiple process
support when I enabled it previously and several parts were broken.
In particular, the resume method was only resuming a single process,
and wait was not stopping other processes when reporting an event.

To support multiple running inferiors, add a new per-inferior
structure which trackes the number of existing and running LWPs for
each process.  The structure also stores a ptid_t describing the
set of LWPs currently resumed for each process.

For the resume method, iterate over all non-exited inferiors resuming
each process matching the passed in ptid rather than only resuming the
current inferior's process for a wildcard ptid.  If a resumed process
has a pending event, don't actually resume the process, but other
matching processes without a pending event are still resumed in case
the later call to the wait method requests an event from one of the
processes without a pending event.

For the wait method, stop other running processes before returning an
event to the core.  When stopping a process, first check to see if an
event is already pending.  If it is, queue the event to be reported
later.  If not, send a SIGSTOP to the process and wait for it to stop.
If the event reported by the wait is not for the SIGSTOP, queue the
event and remember to ignore a future SIGSTOP event for the process.

Note that, unlike the Linux native target, entire processes are
stopped rather than individual LWPs.  In FreeBSD one can only wait on
processes (via pid), not for an event from a specific thread.

Other changes in this commit handle bookkeeping for the per-inferior
data such as migrating the data to the new inferior in the follow_exec
method.  The per-inferior data is created in the attach,
create_inferior, and follow_fork methods.

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