MFC r325436: RTF_PINNED for an interface

MFC r325436: RTF_PINNED for an interface

  Allow a process to assign an IP address to local ppp interface
  even if kernel routing table already has a route to the address in question
  installed by some routing daemon (PR 223129).

  Also, allow loopback route deletion when stopping a VIMAGE jail (PR 222647).

PR:			222647, 223129
Reviewed by:		gnn
Approved by:		mav (mentor)
Differential Revision:	https://reviews.freebsd.org/D12747

git-svn-id: svn://svn.freebsd.org/base/stable/10@326012 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f