Fixed bug where script will hang if 'iconv' hangs, which can happen if
certain Unicode characters exists that may appear in a user's FullName
field.
The fix was to remove the 'iconv' utility entirely, thus eliminating the
risk of a hang. The 'p4 users' command was instead optimized to use
'p4 -ztag -F %User% users' to get just the needed list of users, so that
the FullName fieled is not displayed to begin with (and thus no need for
Unicode tranlsation).
Also changed shebang line to reference python3 rather than python.