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.
Merge branch 'master' of git://git.musl-libc.org/musl into syscall