Fix tzsetup not installing /etc/localtime for UTC

If tzsetup UTC is run then it successfully configured the system for UTC
including installing /etc/localtime however if the user ran just tzsetup
for interactive configuration and select UTC no /etc/localtime was installed
which resulted in failures for utilities which require said file.

Change set_zone_utc to call install_zoneinfo("UTC") to ensure that
/etc/localtime is created for interactive UTC selection.

Users who have previously run tzsetup in interactive mode and select UTC
can install the missing /etc/localtime by running tzsetup -r.

Also correct static miss-match for set_zone_utc.

MFC after: 2 weeks
Relnotes: Yes
Sponsored by: Multiplay