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

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

Remove _SOLARIS_C_SOURCE compatibility definitions. Unfortunately the
ZFS porting style didn't extend this, instead using a heap of additional
header files that don't get installed.

My intention had been to allow OpenSolaris external code to build on
FreeBSD out of the box (i.e. without a src tree).

This file is the same as the one installed in /usr/include, however
there are enough places in the DTrace kernel/module sources that
having a header that gathers together all the individual elf headers
is convenient.

Note that the Solaris compatibility definions are conditionally
included iff _SOLARIS_C_SOURCE is defined.