Add sysctl()

Currently, we have several non-standard OS control interfaces. Some of these are analogous to similar interfaces in NuttX:

- `prctl()`: This is where task/process specific settings should be set. At present, I think this is only the task name. (but I would like to see this extended to include the number of file descriptors in the future).
- ` boardctl():` This was intended to provide hooks into board-specific initialization and control logic.

The problem is that the scope of the `boardctl()` commands has increased. It now includes many commands that are unrelated to boards but are general system control commands. This includes commands to control graphics, ramdisks, symbol tables, power management, etc.

The proposal here is that we create a new system control interface called `sysctl()` as it is in Linux:

- `sysctl()`: This would provide hooks into system-wide initialization and control logic.

Initially this would just move the system-wide commands that are erroneously included in` boardctl()` into `sysctl()`. But this would be a place where we could grow additional system-wide commands.