dlfcn-util: invert function naming and add helper that does the whole job

We warn when the operation fails, not when it succeeds. Hence this should be
"<do>_or_<handle failure>", not "<do>_and_<handle failure>". We *could* use
whatever convention we want, but rust and perl are rather consistent in using
the logical convention. We don't care about perl that much, but having a naming
convention inverted wrt. rust would be rather confusing.

Also, pretty much every implementation does similar steps, so add a nice
wrapper which combines opening of the library and loading of the symbols.

Also add missing sentinel attribute in dlopen_or_warn().