terminal-unix: identify and ignore unknown CSI sequences

If an unknown ESC sequence is detected where an ASCII char <X> follows
the ESC, mpv interprets it as ALT+<X>, which is the traditional
terminal encoding of ALT+letter.

However, if <X> is '[' then it's a CSI sequence which continues after
the '[', and has its own termination rules (can be many chars).

Previously, mpv interpreted unknown CSI sequences as (incorrect) ALT+[
followed by (incorrect) "keys" from the rest of the sequence.

In this commit, if a unknown CSI sequence is detected, mpv ignores
exactly the complete sequence.