To stop GC completely, debug.SetGCPercentage needs to receive a negative
integer. Passing a negative integer is cumbersome:
$ gops setgc <pid> -- -1
This commit adds a more user-friendly way to achieve that:
$ gops setgc <pid> off
This behavior is aligned with that of the GOGC env variable, where
negative or "off" can be set to stop the GC.
Manual command setup was starting to show some strain in
terms of documentation and also adding new features
(regardless of what the protocol supports currently).
This new setup aims to make it easier to add new
documentation and functionality.
In comparison to the previous version, this increased the
binary size by 2.4M.
```
6.4M gops
4.0M gops_master
```
