xorg-server/devd: device_added() takes a const char *

This fixes a compiler warning.
xorg-server/devd: Lower level of "device already opened" message

This situation is expected when using moused(8), so no need to alarm the
user.
xorg-server/devd: In config_devd_fini(), forget we added the console keyboard

This fixes a problem with KDM where the X server isn't restarted after
a session ended, but all input devices were removed. This led to no
keyboard added back when the login/password prompt was presented again
to the user.
xorg-server/devd: Get rid of varargs usage in sysctl_get_str()

This was a bit overkill for a single caller. Furthermore, this fixes a
compiler warning.