Better Upstart conflict handling for serial getty.

Previously, on RHEL-6, we always disabled the automatic starting
of getty on serial port consoles, to avoid conflicts with explicit
configuration of the same port.  This had the disadvantage that even
if logins were not explicit configured for, e.g, ttyS17, and you
booted the system with console=ttyS17, you would not get a login
prompt there.  That is probably not what most users would want.

We here make use of the 'stop on' event handling in Upstart job
definitions, to force the automatic getty job to be stopped, if an
explicit getty job for that port is started.  This assumes that such
jobs are named "serial-$DEV", as the console::serial::login definition
names them.

Signed-off-by: Thomas Bellman <bellman@lysator.liu.se>