add support for RTLD_NOLOAD to dlopen

based on patch by Pierre Carrier <pierre@gcarrier.fr> that just added
the flag constant, but with minimal additional code so that it
actually works as documented. this is a nonstandard option but some
major software (reportedly, Firefox) uses it and it was easy to add
anyway.
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.