replace strncpy() with strlcpy()

The problem was strncpy() doesn't stop after writing the terminating
NUL; by definition it goes on to zero the entire buffer.

I spy another use of strncpy in udev_device_add_property_from_string(),
which is responsible for another ~1% user cpu time...

Signed-off-by: Alan Jenkins <alan-jenkins@tuffmail.co.uk>