Finish <search.h>.

I'm unable to find a bug, but we've had requests for this internally
once or twice (though I pointed those folks at the STL), and there's
code we build for the host or in our bootloaders that would use this,
and there's reasonable-looking FreeBSD implementation ready and waiting.

Bug: N/A
Test: ran tests
Change-Id: I6ddee4b71bea4c22ed015debd31d3eaac4fcdd35