ifconfig: warn if setting an Internet address without mask

Add a postproc function for af_inet, and add interface flags as a
parameter. Check there if setting an address without a mask unless
the interface is loopback or point-to-point, where mask is not really
meaningful; warn if so. This will hopefully be an error in the future.

MFC after: 1 month
Reviewd by: bz
Differential Revision: https://reviews.freebsd.org/D32709