gvc auto_output_filename: avoid 'strdup' when constructing file name

gvc auto_output_filename: avoid 'strdup' when constructing file name

This block of code is scanning a string of `:`-separated entries and writing
them into `buf` in reverse order `.`-separated. We can rewrite it to avoid
dynamic allocation, thus avoiding certain failure cases and locking overhead.

Unfortunately there seems to be no variant of `strrchr` that takes a length. So
we need to write our own loop for locating the last `:` with a limit.