util: add dlfcn-util.h

This just adds a _cleanup_ helper call encapsulating dlclose().

This also means libsystemd-shared is linked against libdl now. I don't
think this is much of an issue, since libdl is part of glibc anyway, and
anything from exotic. It's not an optional part of the OS (think: NSS
requires dynamic linking), hence this pulls in no deps and is almost
certainly loaded into all process' memory anyway.

[zj: use DEFINE_TRIVIAL_CLEANUP_FUNC().]