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

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

It's not enough to provide symbol map files; you also have to add them
to ${SYM_MAPS}. This unbreaks world with SYMVER_ENABLED.

Pointy hat to: ume

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