In contrast to the commandline tool "realpath" the library function
fails for any path pointing to non-existing files (even if the parent
directory exists). This breaks most use-cases for this function.

This reverts commit 1a06687a.

If movs faststart option is set, the muxer will try to reopen the
output file using the path passed to avformat_alloc_output_context2
instead of the one passed to avio_open.