healtcheck: add extra build tags+files for netbsd and openbsd

Without these changes, the disk check portions won't compile on these
platforms.

healthcheck/diskcheck_openbsd.go

@@ -0,0 +1,17 @@
+package healthcheck
+
+import "golang.org/x/sys/unix"
+
+// AvailableDiskSpace returns ratio of available disk space to total capacity
+// for solaris.
+func AvailableDiskSpace(path string) (float64, error) {
+	s := unix.Statfs_t{}
+	err := unix.Statfs(path, &s)
+	if err != nil {
+		return 0, err
+	}
+
+	// Calculate our free blocks/total blocks to get our total ratio of
+	// free blocks.
+	return float64(s.F_bfree) / float64(s.F_blocks), nil
+}