input: set mouse area by default for all input

Otherwise, mouse button bindings added by mp.add_key_binding() would be
ignored.

It's possible that this "breaks" some older scripts using undocumented
Lua script functions, but it should be safe otherwise.

Fixes #1283.
x11: fix crash with --wid=0 and vo_xv/vo_x11

But seriously, don't use --wid=0, don't use vo_xv, and _especially_
don't use vo_x11.

Fixes #1284.
x11: fix crash

Broken by previous commit. Oops.
client API: restrict client names

Use a fixed size array for the client name, which also limits the client
name in size. Sanitize the client name string, and replace characters
that are not in [A-Za-z0-9] with '_'.
input, lua: redo input handling

Much of it is the same, but now there's the possibility to distinguish
key down/up events in the Lua API.
player: don't crash when using sub_seek without subtitles

Recent regression. It turns out the assertion was completely unneeded.

Fixes #1285.
command: don't queue framesteps

If repeated framestep commands are sent, just unpause the player, instead
of playing N frames for N repeated commands.
lua, ipc: remove leftovers

MPV_EVENT_SCRIPT_INPUT_DISPATCH is now unused/deprecated.

Also remove a debug-print from defaults.lua.