options: remove --volstep

It's just confusing; users are encouraged to edit input.conf instead
(changing the argument to the "add" command).

Update input.conf to keep the old behavior.
command: always show OSD message when changing volume etc.

If there's a command that uses the OSD by default, then always print the
associated message (or a fallback made of name + value), even if the
command has an associated OSD bar.

This means volume, gamma, panscan, etc. all show both a message and a
OSD bar.

Also, add a '%' to the volume message. The extra_msg thing is not needed
anymore.

See issue #1103.
command: remove unneeded ifdefs

Why bother?

Also, since now some properties could be mapped to non-existing options,
but mp_property_generic_option() is used, deal with this case and return
a not-found error code.
command: allow changing some OSD-related options even with no VO

If no VO was open, these options couldn't be changed or even queried.
Although these properties are nearly useless if no VO exists, there's
actually no good reason to forbid querying or setting them. Also, even
if the VO is created, it doesn't mean the VO window was created.