BUG/MINOR: haterm: cannot reset default "haterm" mode

BUG/MINOR: haterm: cannot reset default "haterm" mode

When "mode haterm" was set in a "defaults" section, it could not be
overridden in subsequent sections using the "mode" keyword. This is because
the proxy stream instantiation callback was not being reset to the
default stream_new() value.

This could break the stats URI with a configuration such as:

    defaults
        mode haterm
        # ...

    frontend stats
		bind :8181
		mode http
		stats uri /

This patch ensures the ->stream_new_from_sc() proxy callback is reset
to stream_new() when the "mode" keyword is parsed for any mode other
than "haterm".

No need to backport.