um: Add an option to make serial driver non-raw

um: Add an option to make serial driver non-raw

In some cases, for example when the program(s) running inside UML
isn't/aren't interactive (like the hwsim tests for wpa_supplicant)
there's really no value in having the serial lines configured to
be raw as they are now by default. Setting them to non-raw lets
one abort the whole UML with Ctrl-C, which is really the right
thing to do in these cases, basically the whole UML instance is
more like a single (testing) program.

Add a "ssl-non-raw" option to UML to support such a mode.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: Richard Weinberger <richard@nod.at>