Ignore CPR ANSI control sequence in exit_on_escapeseq()

Ignore CPR ANSI control sequence in exit_on_escapeseq()

In some cases (e.g. Debian's initramfs) the device sends DSR (Device
Status Report) ANSI control sequences as a response to pressing Enter.
Some terminals (e.g. gnome terminal) respond to this by CPR (Cursor
Position Response) sequence, which appears as typed from keyboard. See
https://en.wikipedia.org/wiki/ANSI_escape_code#Terminal_output_sequences.

This complicates exiting sterm, because the exit escape sequence \n~.
must by typed very quickly (actually copy&pasted) so that CPR doesn't
appear between \n and ~..

This commit causes the CPR sequence to be ignored when detecting exit
sequence.

Fixes #2.