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

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

Prefix the alias macros for members of struct __mcontext with an underscore
in order to avoid a clash in the net80211 code.

Remove unused variables. Add required headers and function
declarations.

Implement signalcontext.