add RTLD_NODELETE flag for dlopen

this is a trivial no-op, because dlclose never deletes libraries. thus
we might as well have it in the header in case some application wants
it, since we're already providing it anyway.

include/dlfcn.h

@@ -10,6 +10,7 @@ extern "C" {
 #define RTLD_LAZY   1
 #define RTLD_NOW    2
 #define RTLD_NOLOAD 4
+#define RTLD_NODELETE 4096
 #define RTLD_GLOBAL 256
 #define RTLD_LOCAL  0