Add chargen to udp_probe and udp_sweep

This simplifies the checks considerably for PR rapid7/metasploit-framework#2681 from @mcantoni by folding them into the udp_probe and udp_sweep modules, as suggested by @jvazquez-r7. Note, I haven't tested these yet.

## Verification

 - [ ] Configure a target as described in configured in rapid7/metasploit-framework#2681
 - [ ] Run the `udp_probe` module against a host and see the service is detected.
 - [ ] Run the `udp_sweep` module against a host and see the service is detected.

## Rationale for keeping chargen_probe

I think the original chargen_probe module is redudent now, but I can see a case for wanting to be more surgical with your Chargen hunting; I notice that there are other UDP protocols that don't have their own standalone protocol scanners, but I kind of feel like that's more of a bug. If I want to look for citrix endpoints, for example, I **must** use the udp_probe and udp_sweep modules, and I can't configure it to **not** look for these other services. That's kind of weak.

Also, the Description of @mcantoni's module is a nice read. It'd be a shame to lose this risk analysis.