MFC r289315, r292216

resolver: automatically reload /etc/resolv.conf

On each resolver query, use stat(2) to see if the modification time
of /etc/resolv.conf has changed. If so, reload the file and reinitialize
the resolver library. However, only call stat(2) if at least two seconds
have passed since the last call to stat(2), since calling it on every
query could kill performance.

This new behavior is enabled by default. Add a "reload-period" option
to disable it or change the period of the test.

Document this behavior and option in resolv.conf(5).

Polish the man page just enough to appease igor.

Relnotes: yes
Sponsored by: Dell Inc.

Copy head (r256279) to stable/10 as part of the 10.0-RELEASE cycle.

Approved by: re (implicit)
Sponsored by: The FreeBSD Foundation

Nuke some compatibility crufts of resolver for 6.X and earlier.

X-MFC after: never

Don't test if RES_INIT is set to see res_state structure is
initialized. Some application may reset RES_INIT. Use the
way in __res_vinit(), instead.

- Merge our local changes.
- Exclude unnecessary functions for us.