disable system issetugid on OS X since it is not fork-safe

Noticed while testing similar code for AIX.
ok beck@

crypto/compat/issetugid_osx.c

@@ -0,0 +1,16 @@
+/*
+ * issetugid implementation for OS X
+ * Public domain
+ */
+
+#include <unistd.h>
+
+/*
+ * OS X has issetugid, but it is not fork-safe as of version 10.10.
+ * See this Solaris report for test code that fails similarly:
+ * http://mcarpenter.org/blog/2013/01/15/solaris-issetugid%282%29-bug
+ */
+int issetugid(void)
+{
+	return 1;
+}