pty: Move filtering escape characters to the PTY

pty: Move filtering escape characters to the PTY

This is probably the place where this should live. We are now processing
cursor positioning events inside the same line and we are filtering out
any escape sequences that we don't know.

It is a bit sad to lose the colors, but I don't have to have two places
where we process this. At least this is going to be more robust and we
can be sure to not forward anything malicious to the controlling
terminal.

Signed-off-by: Michael Tremer <michael.tremer@ipfire.org>