x11_common: uncrustify

Also some other cosmetic changes. And reformat the two remaining doxygen
comments.

Removing MSGLEN in x11_errorhandler() is technically not just a cosmetic
change, but the result is the same anyway.
core: move `xineramascreen` to `MPOpts` as `vo_screen_id`

This is a small cleanup in preparation for the next commit.
core: add fs-screen option for fullscreen display selection

`--fs-screen` allows to decide what display to go fullscreen into. The
semantics of `--screen` changed and now it is only used to select the windowed
display when starting the application.

This is useful for people using mpv with an external TV. They will start
windowed on their laptop's screen and switch to fullscreen on the TV.

@wm4 worked on the x11 and w32 parts of the code. All is squashed in one
commit for history clarity.