kmscon: use /bin/login -p as default login program

We must pass "-p" to make "login" preserve the environment. We make sure
the environment is correctly reset before calling it so it is safe.
Furthermore, this allows us to pass additional environment variables to
the child without having /bin/login reset it.

Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>

docs/man/kmscon.xml

@@ -224,7 +224,7 @@
                 after all options have been parsed is used as argument to this
                 option. Note that nothing after two dashes <option>--</option>
                 is parsed as regular option by kmscon.
-                (default: /bin/login)</para>
+                (default: /bin/login -p)</para>
 
           <para>This example starts '/bin/bash -i' on each new terminal session:
                 ./kmscon --login --debug --no-switchvt -- /bin/bash -i</para>