Silence some compiler warnings

None of these were actual issues.
m_option: fix -vf-del in profiles

We don't bother with option verification (as it happens in profiles),
because it's barely possible.
m_option: fix parameter comparison for vf-toggle

The vf-toggle option parsing (normally used for runtime video filter
switching only) was missing comparing the parameter values. Fix this,
and also make the code a bit more robust.
vf_lavfi: remove redundant statements
configure: map --enable-sdl2 to autodetection

Commit 02bbd87b disabled SDL linking by default. This commit followed
the ancient mplayer convention of disabling detection of compiler flags
with  --enable-* switches. Unfortunately, this makes compiling with SDL
enabled a pain.

Make --enable-sdl/sdl2 use autodetection, even if it's inconsistent with
most other --enable-* switches. The same is already done for
--enable-openal, though.

Based on a pull request by qyot27.
mplayer: re-add some legacy slave mode output for issue #92

In the long run this should be done differently. ID_... output sucks.
This commit will be reverted as soon as I have a good idea how this
should be done properly.
dec_video: remove emms usage

libavcodec generally shouldn't have this problem anymore (if libavcodec
ever had it). All other video decoders are gone. In any case, if this
commit actually causes regressions, these are libavcodec bugs and should
be fixed there instead.
dec_video: get rid of two global variables
options: remove dead -dr1 option