lavf: add directory listing API

TODO: bump minor, update doc/APIchanges

API allows protocol implementations to provide API that
allows to list directory content.
API is similar to POSIX opendir/readdir/closedir.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/libssh: implement directory listing callbacks

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/ftp: implement directory listing callbacks

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavu/avstring: add av_append_path_component() funcion

TODO: bump minor, update doc/APIchanges

Convinient function to build paths.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
lavf/file: implement directory listing callbacks

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
doc/examples: add directory listing example