avconv: simplify exit_program() by using more local vars
avconv: rename InputStream.opts to InputStream.decoder_opts

This makes it more clear what is this variable for exactly.
avconv: rename 'icodec' to 'dec_ctx'

This more clearly describes what is the variable used for.
avconv: rename 'codec' to 'enc_ctx'

This more clearly describes what is the variable used for.

Also, move its declaration into the block where it is actually used.
avconv: remove a useless line

Doing anything with that field when we are doing streamcopy (so the
encoder is not opened) should have absolutely no effect.
avconv: rename OutputStream.opts to OutputStream.encoder_opts

This makes it more clear what is this variable for exactly.