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: avg (mentor), mav (mentor)
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D12747