configure: check for socat and its escape feature

Currently we use a static value for the socat path, or we trust the
user-provided one. With this patch we still trust any user provided
value, but if none is passed we check for socat on the machine we're
being configured on. This allows us also to check if we can or cannot
use the escape= feature in socat.

If the user has forced the path in, he can also pass --with-socat-escape
in order to force the escape functionality to be used, even if a check
is not done.

Signed-off-by: Guido Trotter <ultrotter@google.com>
Reviewed-by: Iustin Pop <iustin@google.com>