lavd/alsa: implement get_device_list callbacks

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
ffserver_config: postpone codec context creation

So far AVCodecContext was created without codec specified.
This causes internal data to not be initialized to defaults.

This commit postpone context creation until all information is gathered.

Partially fixes #1275
ffserver_config: fix compilation warning

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>