avserver: do not use a static string as a default for a string option

Fixes an invalid free.

Bug-id: 448
avconv: fix usage of deprecated lavfi API
lavfi/fifo: fix flushing when using request_samples

If any samples are still buffered when request_frame returns EOF, they
won't be returned currently.
avconv: insert extra filters in the same way for both graph inputs and outputs

This will allow using the same code when one filter can be inserted both
on input and output.
avconv: distinguish between -ss 0 and -ss not being used

Using -ss 0 to drop frames with negative timestamps is a perfectly valid
use case.
avconv: make input -ss accurate when transcoding

Insert (a)trim filters on the corresponding inputs, so the extra frames
are decoded and discarded.
avconv: support -t as an input option.

It limits the duration of the data read from a given input.