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().]

src/basic/meson.build

@@ -39,6 +39,7 @@ basic_sources = files('''
         device-nodes.h
         dirent-util.c
         dirent-util.h
+        dlfcn-util.h
         efivars.c
         efivars.h
         env-file.c
@@ -328,7 +329,8 @@ libbasic = static_library(
                         threads,
                         libcap,
                         libselinux,
-                        libm],
+                        libm,
+                        libdl],
         c_args : ['-fvisibility=default'],
         install : false)