rc.d/random: bring back rc.conf defaults, minor fixes

 * Saving the entropy across reboots is good practice to
   avoid having a lack of entropy after restarting the
   machine. For some reason, it has been disabled by default
   for ages now.

 * This commit brings back the defaults in rc.conf to enable
   saving entropy across reboots.

 * Minor fixes to rc.d/random, such as not assuming
   /var/db/entropy is a good place to put the entropy file,
   last but not least because:
    - that location is not checked on the start part of the script,
      so it won't ever be used.
    - /var/db/entropy is most of the time a directory, for the
      cron-based entropy collection.
kernel - Fix stability issue with net.inet.tcp.inflight*

* net.inet.tcp.inflight_enable is (already) turned on by default.  This
  is necessary for network stability and to reduce excessive packet buffering
  to make PF and other routing protocols on the network work better.

* There is a bug at very high bandwidths where the 'bw' calculation is
  unstable and can lead to positive feedback instead of the desired negative
  feedback, preventing the tcp buffer size from ramping up properly.
  GigE links could sometimes run a tcp stream as slow as 7 MBytes/sec due
  to the bug due to bwnd limiting the tcp buffer to ~30-50KB.

* Fix the bug by increasing the minimum delta ticks for calculating (bw)
  from 1 to 2 and changing the stab parameter to scale bw instead of adding a
  fixed number of tcp maxsegs to bwnd.  This handles the high-speed issue.
  Low speed stability issues are handled by also adding 2*maxseg to bwnd.

  With this fix, GigE links stabilize better at the ~400-450KB needed to run
  the stream at full speed.

* net.inet.tcp.inflight_stab now scales bw in 1/10% increments instead
  of adding N*tcp_maxseg/10.  The default value of 50 increases the 'bw'
  calculation used to derive bwnd by 5%.  This appears to be sufficient.

Reported-by: alexh, rob__
powerd - Set to max performance if killed

* If the powerd daemon is killed it now sets the system to maximum
  performance before exiting for consistency, instead of leaving it
  in a potentially low-power state.
kernel/netgraph7: Sync ng_bt3c with current FreeBSD and port it over.

I can't test this hardware unfortunately since I don't own it.