Improve the ANSI sequence parsing/stripping/pass-through in spy()

Strip all *but* the expected keyboard input sequences (arrow keys, pgup/dn,
home, end, insert). Pass-through a bare ESC key (albeit with delay). ESC
immediately followed by a key other than '[' should be very quickly passed
through.
Increase the max ANSI sequence length from 31 to 256 chars.