m_option: redo code for parsing -vf to accept quotes

Parsing sub-configs (like --rawvideo=subopts or the suboptions for
--vo=opengl:subopts) was completely different from the -vf parsing code
for a variety of reasons. This change at least makes -vf use the same
splitter code as sub-config options.

The main improvement is that -vf now accepts quotes, so you can write
things like:

     -vf 'lavfi=graph="gradfun=10:20"'

(The '' quotes are for shell escaping.)

This is a rather big and intrusive change. Trying some -vf lines from
etc/encoding-example-profiles.conf seems to confirm it still works.

This also attempts to unify one subtle difference in handling of
positional arguments. One consequence is that a minor detail changes.
Sub-configs don't know positional arguments, and something like "--
opt=sub1=val1:sub2" means that sub2 has to be a flag option. In -vf
parsing, sub2 would be a positional option value. To remove this
conflict and to facilitate actual unification of the parsers in the
future, the sub2 will be considered a flag option if and only if such a
flag option exists. Otherwise, it's considered a value for a positional
option.

E.g. if there's a filter "foo" with a string option "sopt" and a flag
option "fopt", the behavior of the following changes:

   -vf foo=fopt

Before this commit, this would set "sopt=fopt" in the filter. Now, it
enables the fopt flag, and the sopt option remains unset. This is not an
actual problem to my knowledge.

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