sys/netinet6: Implement RFC 7217HEADmain

Implement RFC 7217 (A Method for Generating Semantically Opaque
Interface Identifiers with IPv6 Stateless Address Autoconfiguration
(SLAAC)) in our IPv6 stack.

A new ifconfig `stableaddr` flag is added to enable the feature on
interfaces, which defaults to on or off for new interfaces based
on the sysctl `net.inet6.ip6.use_stableaddr` (off by default, so
this commit causes no change in behavior with default settings).

The algorithm follows the RFC in its logic, using SHA256-HMAC as
the algorithm to derive addresses so as to provide code that can
be leveraged by future implentations of RFC 8981, leveraging the
`hostuuid` as the secret.

The source of the hostidentifier can be configured using the sysctl
`net.inet6.ip6.stableaddr_netifsource`, while the number of retries
generating a new address in case of collision can be configured
using the `net.inet6.ip6.stableaddr_maxretries` sysctl (default 3).

Documentation about all these flags is added to the ifconfig(8) man
page.

Reviewed by:		cognet, glebius, hrs
Tested by:		zarychtam@plan-b.pwste.edu.pl
Approved by:		cognet, glebius
Relnotes:		yes
Differential Revision:	https://reviews.freebsd.org/D49681

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