UPDATING: Chase sysutils/screen-4.9.0_3

Describe user impact as a result of sysutils/screen-4.9.0_3 which removes
setuid root by default, disabling multiuser feature. The option is
appropriately called MULTUSER. Users may enable the multiuser feature
in three ways, as discsussed by the update to UPDATING.

PR: 262903
Reported by: david@isnic.is