Fix setMaxThreads in http-wordpress-brute NSE script

[Nukesor]

The setMaxThreads function requires an Int as parameter as the nselib/brute.lua will die, if we provide a string: https://github.com/nmap/nmap/blob/master/nselib/brute.lua#L1188

The script sets the max threads number after the Engine initialization, but uses the parameter string without converting it to an int.

[dmiller-nmap]

Thanks! I found several other places where this could cause problems, so I fixed those as well and credited you. I want point out that http-form-brute should be capable of brute-forcing Wordpress installations since 7.10. I'd appreciate your feedback on any problems you find in that script.