Fix dst/netmask handling in routing socket code.

Traditionally routing socket code did almost zero checks on
the input message except for the most basic size checks.

This resulted in the unclear KPI boundary for the routing system code
(rtrequest* and now rib_action()) w.r.t message validness.

Multiple potential problems and nuances exists:

• Host bits in RTAX_DST sockaddr. Existing applications do send prefixes with hostbits uncleared. Even route(8) does this, as they hope the kernel would do the job of fixing it. Code inside rib_action() needs to handle it on its own (see rt_maskedcopy() ugly hack).
• There are multiple way of adding the host route: it can be DST without netmask or DST with /32(/128) netmask. Also, RTF_HOST has to be set correspondingly. Currently, these 2 options create 2 DIFFERENT routes in the kernel.
• no sockaddr length/content checking for the "secondary" fields exists: nothing stops rtsock application to send sockaddr_in with length of 25 (instead of 16). Kernel will accept it, install to RIB as is and propagate to all rtsock consumers, potentially triggering bugs in their code. Same goes for sin_port, sin_zero, etc.

The goal of this change is to make rtsock verify all sockaddr and prefix consistency.
Said differently, rib_action() or internals should NOT require to change any of the
sockaddrs supplied by rt_addrinfo structure due to incorrectness.

To be more specific, this change implements the following:

• sockaddr cleanup/validation check is added immediately after getting sockaddrs from rtm.
• Per-family dst/netmask checks clears host bits in dst and zeros all dst/netmask "secondary" fields.
• The same netmask checking code converts /32(/128) netmasks to "host" route case (NULL netmask, RTF_HOST), removing the dualism.
• Instead of allowing ANY "known" sockaddr families (0<..<AF_MAX), allow only actually supported ones (inet, inet6, link).
• Automatically convert sockaddr_sdl (AF_LINK) gateways to sockaddr_sdl_short.

Reported by: Guy Yur <guyyur at gmail.com>
Reviewed By: donner
Differential Revision: https://reviews.freebsd.org/D28668
MFC after: 3 days