command: fix compilation with MinGW

This include header is needed for the fork/exec code, which is inactive
on Windows anyway.
manpage: mark "run" command as Unix-only
mixer: remove comment about af_pan doing downmixing

We don't do that anymore.
manpage: remove some --flip leftovers
vf: print error when creating filter fails

Before that we relied on the filters printing their own error messages.
vf: remove unused function
video/filter: remove vf_match_csp()

This function improves automatic filter insertion, but this really
should be done by the generic filter code. Remove vf_match_csp() and all
code using it as preparation for that.

This commit temporarily makes handling of filter insertion worse for
now, but it will be fixed with the following commits.
video: create a separate context for video filter chain

This adds vf_chain, which unlike vf_instance refers to the filter chain
as a whole. This makes the filter API less awkward, and will allow
handling format negotiation better.
vf: remove flags from filter format status

I don't think we need these flags anymore. Simplify the code and get rid
of the vf_format struct.

There still is the vf_format.configured field, but this can be replaced
by checking for a valid image format.
video/filter: make vf->control non-recursive

Reason: I never liked it being recursive. Generally, this seems to
cause more problems than trouble, and is less flexible for access
outside of the chain.
vf: redo conversion filter insertion/format negotiation

Remove the inconsistent, duplicated, and insufficient scale filter
insertion code, and do it in one place instead. This also compensates
for the earlier removal of vf_match_csp() (which was in fact duplicated
code).

The algorithm to determine where to insert a filter etc. is probably the
same, though it also comes with some changes that should make debugging
easier when trying to figure out why a chain is failing to configure.

Add an "in" pseudo filter, which makes insertion of conversion filters
easier. Also change the vf->reconfig signature. At a later point, I'll
probably change format negotiation such that the generic filter code
will choose the output format, so having separate in and out params will
be useful.
options: allow hwaccel formats in -vf format/noformat

There are 3 users of the image format option type: demux_raw,
vf_format, vf_noformat. Allow the hwaccel formats (like vdpau etc.)
in general, so that the filters can use it. This won't work for
demux_raw, so explicitly reject these formats there.