ffserver: move configuration code to separate file

This commit doesn't change any existing logic.
It moves ffserver configuration related code to separate file.
It intends to make maintaining easier.

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
ffserver_config: clean up variable initialization

Signed-off-by: Lukasz Marek <lukasz.m.luki2@gmail.com>
ffserver_conf: factorize parse function per config tag

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