Make sure we do not pass multiple CGI parameters in function calls

Make sure we do not pass multiple CGI parameters in function calls

When CGI->param is called in list context, such as in function
parameters, it expands to all the potentially multiple values
of the parameter: for instance, if we parse query string a=b&a=c&d=e
and call func($cgi->param('a')), that's equivalent to func('b', 'c').
Most of the functions we're calling do not expect that.

I do not believe this is an exploitable security vulnerability in
ikiwiki, but it was exploitable in Bugzilla.