debian/postinst: Use '--system' to create a system username (UID < 1000)
debian/postinst: Use '--system' to create a system username (UID < 1000)

Without '--system', the user gets created with an UID > 1000 (typically
1001 given that Kurento tends to be installed in clean machines, and it
gets the second available UID).

All users with UID >= 1000 show up in the Ubuntu login screen, which is
not something desired for the kurento user.

Reported here:
https://groups.google.com/d/msg/kurento/grivHwgAVTI/tgLPUGs4CQAJ

Kurento/kurento-media-server,Kurento/kurento-media-server