update-users-groups.pl: Use UTF-8 instead of latin1.

Perl seems to write the file in latin1 independent of the actual input
encoding.  This can corrupt the `description` field of `/etc/passwd`.  By
setting `binmode` to `:utf8` Perl can be forced to write UTF-8.  Ideally
the program would simply read/write the fields by value without any
changes in encoding.  However, assuming/enforcing UTF-8 is a lot better
than using an obsolete coding like latin1.