Partially implement the mysterious cons25 \e[x escape sequence.

Partially implement the mysterious cons25 \e[x escape sequence.

It seems the terminfo library on some systems (OS X, Linux) may emit the
sequence \e[x to reset to default attributes. Apart from using the
zero-command, this escape sequence allows many more operations, such as
setting ANSI colors. I don't see this used anywhere, so this should be
sufficient for now.

This deficiency was spotted by the Debian GNU/kFreeBSD. They have their
own patch, which is slightly flawed in my opinion. I don't know why they
never reported this issue to us.

MFC after:	1 week