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/shared/mount-util.c

@@ -451,7 +451,7 @@ int bind_remount_one_with_mountinfo(
 
         rewind(proc_self_mountinfo);
 
-        r = dlopen_libmount();
+        r = dlopen_libmount(LOG_DEBUG);
         if (r < 0)
                 return r;
 
@@ -900,7 +900,7 @@ int mount_option_mangle(
          * The validity of options stored in '*ret_remaining_options' is not checked.
          * If 'options' is NULL, this just copies 'mount_flags' to *ret_mount_flags. */
 
-        r = dlopen_libmount();
+        r = dlopen_libmount(LOG_DEBUG);
         if (r < 0)
                 return r;