parser-cfg: use bstr everywhere after parsing stage

Until now it used both char[] and bstr variants in the same code, which
was nasty. For example, the next commit would have additionally required
using memmove() to remove the prefix from the char[] string.
parser-cfg: allow putting options with leading "--"

Now you can pretend the config file is quite literally command line
values dumped into a file, e.g.

--option1=value
--option2=value
...

although the underlying mechanisms are quite different.
example.conf: add example for disabling the OSC
osc: use "info" instead of "warn" for config file creation message

When the main mpv config file, "info" is used, so this makes things
consistent.