vd_lavc: make auto the same as auto-safe and introduce auto-unsafe

Users basically never need --hwdec=auto in reality and sometimes it
bites them unexpectedly (e.g. #15814). Unfortunately the name is kind of
nice sounding and if you don't read the manual closely, you might
inadvertently end up using it. So just make auto the same as auto-safe.
auto-unsafe is introduced to have the old behavior of auto instead. A
similar thing is done with the -copy variants. Try to update the
documentation of this section a little bit and discourage people from
using auto-unsafe because it's kind of dumb.