options: let unknown option case be handled by final option parser

If an option is completely missing, let m_config_parse_option() handle
this case, instead of erroring out early.

Needed for the following commit.
options: print replacement for renamed/replaced options

Basically, extract the option table from DOCS/man/en/changes.rst, and
search the table if an option wasn't found. If there's an entry about
it, print it. Hopefully this behavior is slightly more userfriendly.

This is strictly bound to option names. It doesn't work for option
values, nor does it attempt to emulate the old option.
common: change mp_snprintf_append semantics

Make it more suitable for chaining. This means a function formatting a
value to a string using a static buffer can work exactly like
mp_snprintf_append itself.

Also rename it to mp_snprintf_cat, because that's shorter.
vo: get rid of config_count field

Doesn't really seem to be much of use. Get rid of the remaining uses of
it.

Concerning vo_opengl_old, it seems uninitGl() works fine even if called
before initialization.
vo: don't crash when changing panscan before first configuration

When the VO was not initialized with vo_reconfig(), or if the last
vo_reconfig() failed, changing panscan would cause a crash due to
vo_get_src_dst_rects() dereferencing vo->params (NULL if not
configured).

Just do nothing if that happens, as there is no video that could be
displayed anyway.