vd_lavc: make option struct local

Removes specifics from options.h and options.c, and puts everything into
vd_lavc.c.
ad_lavc: make option struct local

Similar to previous commit.
demux_lavf: make option struct local

Similar to previous commits.
input: make option struct local

Similar to previous commits.

This also renames --doubleclick-time to --input-doubleclick-time, and
--key-fifo-size to --input-key-fifo-size. We could keep the old names,
but these options are very obscure, and renaming them seems better for
consistency.
encode: make option struct local

Similar to previous commits.