runtime: rename close to closefd

Avoids shadowing the builtin channel close function.

Change-Id: I7a729b0937c8248fe27222be61318a88db995eee
Reviewed-on: https://go-review.googlesource.com/8898
Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>

src/runtime/os1_openbsd.go

@@ -133,7 +133,7 @@ var urandom_dev = []byte("/dev/urandom\x00")
 func getRandomData(r []byte) {
 	fd := open(&urandom_dev[0], 0 /* O_RDONLY */, 0)
 	n := read(fd, unsafe.Pointer(&r[0]), int32(len(r)))
-	close(fd)
+	closefd(fd)
 	extendRandom(r, int(n))
 }