debian/postinst: register new home dir with usermod
debian/postinst: register new home dir with usermod

When a new home is created while migrating from "/var/kurento" to
"/var/lib/kurento", not only the new dir has to be created, but the
system file "/etc/passwd" must be updated accordingly. This is done via
the usermod command.

Cross-repo: kms-core, kurento-media-server

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