command: simplify OSD property display code

Probably not many user-visible changes. One notable change is that the
terminal OSD code for OSD bar fallback handling is removed with no
replacement. Instead, terminal OSD gets the same text message as normal
OSD. For volume, this is ok, because the text message is reasonable.
Other properties will look worse, but could be adjusted, and there are
in fact no other such properties that would be useful in audio-only
mode.

The fallback message for seeking falls away as well, but that message
was useless anyway - the terminal status line provides all information
anyway.

I believe the show_property_osd() code is now much easier to follow.
command: when changing a property, always show it on OSD

This means that if a property not listed in property_osd_display[] is
changed, it will be shown on the OSD as "name: ${name}".

Properties that are listed in property_osd_display[] and have osd_name
not set stay invisible by default. This is used for "pause" and
"fullscreen", which (like before this commit) are not shown by default,
because it would be annoying.

The defaults still can be changed with command prefixes (osd-msg,
no-osd, others).
command: use show_property_osd() sub_step

Simpler and more consistent.
command: improve "osd" command

Be less annoying, print the actual OSD level instead of something
meaningless, but still clear the OSD if OSD level 0 (no OSD) is set.

Remove the special handling for terminal OSD, that was just dumb.
osd: fix --term-osd=no

It didn't actually disable it.

I'm actually not sure what this option is useful for, but fixing it
can't harm.