input, lua: add functions to take pre-split input commands

So you can pass a command as list of strings (each item is an argument),
instead of having to worry about escaping and such.

These functions also take an argument for the default command flags. In
particular, this allows setting saner defaults for commands sent by
program code.

Expose this to Lua as mp.send_commandv command (suggestions for a better
name welcome). The Lua version doesn't allow setting the default command
flags, but it can still use command prefixes. The default flags are
different from input.conf, and disable OSD and property expansion.
osc: use mp.send_commandv

See previous commit. Drop no-osd prefixes, as this is the default for
send_commandv anyway. send_command should probably be renamed later.