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/basic/tmpfile-util.h

@@ -8,6 +8,12 @@ int fopen_temporary_at(int dir_fd, const char *path, FILE **ret_file, char **ret
 static inline int fopen_temporary(const char *path, FILE **ret_file, char **ret_path) {
         return fopen_temporary_at(AT_FDCWD, path, ret_file, ret_path);
 }
+
+int fopen_temporary_child_at(int dir_fd, const char *path, FILE **ret_file, char **ret_path);
+static inline int fopen_temporary_child(const char *path, FILE **ret_file, char **ret_path) {
+        return fopen_temporary_child_at(AT_FDCWD, path, ret_file, ret_path);
+}
+
 int mkostemp_safe(char *pattern);
 int fmkostemp_safe(char *pattern, const char *mode, FILE**_f);