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

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>