avconv: print an error on applying options of the wrong type.

I.e. input options to output files or vice versa.
avconv: use a local variable for OutputFile in open_output_file().

This is shorter and more readable.
avconv: use a local variable for InputFile in open_input_file().

This is shorter and more readable.
FATE: remove the -dct option from some decoding tests.

It is an encoding option, it has no effect when applied to decoding.
FATE: replace -vb with -b:v

The -{vas}<option> syntax is now obsolete and deprecated, and has never
been documented or encouraged.
avconv: fix a typo
avconv: do not silently ignore unused codec AVOptions.

Print an error and abort when the option is of the wrong type (decoding
for output file or vice versa), since this could never be correct for
any input or output configuration.

Print a warning and continue when the option is of the correct type,
just unused, so same commandlines can be reused for different kinds of
input or output files.