m_option: allow quoted positional parameters for -vf

This allows things like:

    '--vf=lavfi="gradfun=20:30"'

Adjust the documentation for vf_lavfi to make the example less verbose.
As an unrelated change, add a general description to vf_lavfi.
configure: enable libavdevice by default

Used to be disabled by default, because libavdevice depends on
libavfilter, and earlier versions of libavfilter exports symbols that
clash with mpv's due to its MPlayer filter wrapper. Our configure script
explicitly detects these symbols now, and we can use that to safely
auto-detect libavdevice too. If we detect that libavfilter can't be
safely used, libavdevice is disabled as well.
manpage: remove references to --no-slices

Background: slice support has been completely removed, because it
doesn't work with multithreading, and provides a rather bad complexity
to performance tradeoff otherwise.
vo_opengl_old: remove nomanyfmts warning

The VO warns by default that the nomanyfmts option should be used if
video display fails. This is almost completely useless, but people
keep asking what it means.
m_option: fix positional sub-option skipping

Empty sub-option parameters mean the sub-option should be skipped,
e.g. -vf gradfun=:10 sets the second option (by position) to 10. This
was broken in commit 04f1e2d.