Merge 260488, r260508.

r260488:
  Split rt_newaddrmsg_fib() into two different functions.
  Adding/deleting interface addresses involves access to 3 different subsystems,
  int different parts of code. Each call can fail, so reporting successful
  operation by rtsock in the middle of the process error-prone.

  Further split routing notification API and actual rtsock calls via creating
  public-available rt_addrmsg() / rt_routemsg() functions with "private"
  rtsock_* backend.

r260508:
  Simplify inet alias handling code: if we're adding/removing alias which
  has the same prefix as some other alias on the same interface, use
  newly-added rt_addrmsg() instead of hand-rolled in_addralias_rtmsg().

  This eliminates the following rtsock messages:

  Pinned RTM_ADD for prefix (for alias addition).
  Pinned RTM_DELETE for prefix (for alias withdrawal).

  Example (got 10.0.0.1/24 on vlan4, playing with 10.0.0.2/24):

  before commit, addition:

    got message of size 116 on Fri Jan 10 14:13:15 2014
    RTM_NEWADDR: address being added to iface: len 116, metric 0, flags:
    sockaddrs: <NETMASK,IFP,IFA,BRD>
     255.255.255.0 vlan4:8.0.27.c5.29.d4 10.0.0.2 10.0.0.255

    got message of size 192 on Fri Jan 10 14:13:15 2014
    RTM_ADD: Add Route: len 192, pid: 0, seq 0, errno 0, flags:<UP,PINNED>
    locks:  inits:
    sockaddrs: <DST,GATEWAY,NETMASK>
     10.0.0.0 10.0.0.2 (255) ffff ffff ff

  after commit, addition:

    got message of size 116 on Fri Jan 10 13:56:26 2014
    RTM_NEWADDR: address being added to iface: len 116, metric 0, flags:
    sockaddrs: <NETMASK,IFP,IFA,BRD>
     255.255.255.0 vlan4:8.0.27.c5.29.d4 14.0.0.2 14.0.0.255

  before commit, wihdrawal:

    got message of size 192 on Fri Jan 10 13:58:59 2014
    RTM_DELETE: Delete Route: len 192, pid: 0, seq 0, errno 0, flags:<UP,PINNED>
    locks:  inits:
    sockaddrs: <DST,GATEWAY,NETMASK>
     10.0.0.0 10.0.0.2 (255) ffff ffff ff

    got message of size 116 on Fri Jan 10 13:58:59 2014
    RTM_DELADDR: address being removed from iface: len 116, metric 0, flags:
    sockaddrs: <NETMASK,IFP,IFA,BRD>
     255.255.255.0 vlan4:8.0.27.c5.29.d4 10.0.0.2 10.0.0.255

  adter commit, withdrawal:

    got message of size 116 on Fri Jan 10 14:14:11 2014
    RTM_DELADDR: address being removed from iface: len 116, metric 0, flags:
    sockaddrs: <NETMASK,IFP,IFA,BRD>
     255.255.255.0 vlan4:8.0.27.c5.29.d4 10.0.0.2 10.0.0.255

  Sending both RTM_ADD/RTM_DELETE messages to rtsock is completely wrong
  (and requires some hacks to keep prefix in route table on RTM_DELETE).

  I've tested this change with quagga (no change) and bird (*).

  bird alias handling is already broken in *BSD sysdep code, so nothing
  changes here, too.

  I'm going to MFC this change if there will be no complains about behavior
  change.

  While here, fix some style(9) bugs introduced by r260488
  (pointed by glebius and bde).