libc: Reduce -Wmissing-prototypes warnings a bit.

libc: Use _ioctl() internally.

Sync lib/net with FreeBSD:

* Add a byteorder.9 manual page.
* Remove the addr2ascii and ascii2addr functions. They aren't used in
the tree, and there's other functions that are more generic and

Sync lib/net with FreeBSD:

* Add a byteorder.9 manual page.
* Remove the addr2ascii and ascii2addr functions. They aren't used in
the tree, and there's other functions that are more generic and
appropriate.
* Fix a few style issues.
* Add and document ether_ntoa_r() and ether_aton_r() functions, which
accept passed storage buffers rather than using static storage.
Reimplement ether_ntoa() and ether_aton() in terms of these functions.
These variants are thread-safe.
* Document that AI_ALL and AI_V4MAPPED flags are currently not supported.
* Add a SIOCGIFINDEX ioctl, which returns the index of a named interface.
Use this ioctl in if_nametoindex(3) to use network aliases.
* Add restrict qualifier where needed.
* Add inet_ntoa_r() - a re-entrant function of inet_ntoa.
* Note that inet6_option_space.3 and inet6_rthdr_space.3 functions are
deprecated in favor of inet6_opt_init.3 and inet6_rth_space.3.
* Remove NS and ISO stuff.
* Sync map_v4v6.c with BIND9.
* Instead of re-implementing hton[ls] and friends for each arch, add a
new MI file, net/ntoh.c, which just implement them using the inline
functions from <sys/endian.h>.
* Fix some namespace issues in <netdb.h>.

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