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

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

People porting FreeBSD to new architectures ought not have to
implement a deprecated FPU control interface in addition to the
standard one. To make this clearer, further deprecate ieeefp.h
by not declaring the function prototypes except on architectures
that implement them already.

Currently i386 and amd64 implement the ieeefp.h interface for
compatibility, and for fp[gs]etprec(), which doesn't exist on
most other hardware. Powerpc, sparc64, and ia64 partially implement
it and probably shouldn't, and other architectures don't implement it
at all.

/* -> /*- for license, minor formatting changes

Use the definitions in machine/fsr.h instead of duplicating these magic
numbers here (the values need to correspond to the %fsr ones for some
libc functions to work right).

Standardize idempotentcy ifdefs.

Add kernel headers needed to build libc. Some are bogus and/or just enough
to compile.

Mostly obtained from: netbsd