dlopen: take log_level argument and log in fallback stubs

Every dlopen_xxx() helper now takes an int log_level argument. It is
passed through to dlopen_many_sym_or_warn() (which in turn propagates it
to dlopen_verbose() for the library-not-installed case), and is used by
the fallback stub when support for the library is not compiled in to
emit a "<lib> support is not compiled in." message at the caller's level.
Callers pass LOG_DEBUG when gracefully degrading, or a higher level when
the failure should surface, and no longer need to log redundantly at the
call site.

As part of this, dlopen_bpf_full() (which already took a log_level) is
merged into dlopen_bpf() rather than keeping both.

The static inline fallbacks used to live in the headers, which required
pulling log.h in from every header that declared a dlopen_xxx(). Move
them into the .c files instead: the declaration is always outside the
#if HAVE_XXX block, the impl sits outside the outer #if HAVE_XXX wrap
with its own internal #if HAVE_XXX/#else/#endif, and apparmor-util.c,
idn-util.c, libmount-util.c and pam-util.c are now always compiled so
they can host their stubs.

src/nspawn/nspawn-oci.c

@@ -1826,7 +1826,7 @@ static int oci_seccomp(const char *name, sd_json_variant *v, sd_json_dispatch_fl
         if (r < 0)
                 return json_log(def, flags, r, "Unknown default action: %s", sd_json_variant_string(def));
 
-        r = dlopen_libseccomp();
+        r = dlopen_libseccomp(LOG_DEBUG);
         if (r < 0)
                 return json_log(def, flags, r, "No support for libseccomp: %m");
 

src/nspawn/nspawn.c

@@ -5961,9 +5961,9 @@ static int run(int argc, char *argv[]) {
         if (arg_cleanup)
                 return do_cleanup();
 
-        (void) dlopen_libmount();
-        (void) dlopen_libseccomp();
-        (void) dlopen_libselinux();
+        (void) dlopen_libmount(LOG_DEBUG);
+        (void) dlopen_libseccomp(LOG_DEBUG);
+        (void) dlopen_libselinux(LOG_DEBUG);
 
         r = cg_has_legacy();
         if (r < 0)