tmpfile-util: Introduce fopen_temporary_child()

Instead of having fopen_temporary() create the file either next
to an existing file or in tmp/, let's split this up clearly into
two different functions, one for creating temporary files next to
existing files, and one for creating a temporary file in a directory.

src/shared/mkfs-util.c

@@ -254,7 +254,7 @@ static int make_protofile(const char *root, char **ret) {
 
         assert(ret);
 
-        r = fopen_temporary(NULL, &f, &p);
+        r = fopen_temporary_child(NULL, &f, &p);
         if (r < 0)
                 return log_error_errno(r, "Failed to open temporary file: %m");
 

src/shared/tmpfile-util-label.c

@@ -14,6 +14,8 @@ int fopen_temporary_label(
 
         int r;
 
+        assert(path);
+
         r = mac_selinux_create_file_prepare(target, S_IFREG);
         if (r < 0)
                 return r;