Namespace our KCL args, organize parsing

[zdykstra]

Move all ZFSBootMenu-specific arguments to the zbm.* argument space. Legacy values are still accepted in lieu of the new zbm. value, but will produce a warning on the kernel ring buffer. Timeout values have been expanded to work as follows:

zbm.timeout (or timeout) to set an explicit timeout option
OR zbm.show sets menu_timeout=-1 to force the main menu
OR zbm.skip sets menu_timeout=0 to skip the menu and directly boot BOOTFS
finally if none of these options are set, menu_timeout=10 is set.

The order of these tests might want to be flipped around to give precedence to zbm.show / zbm.skip - thoughts?

[ahesford]

I don't think the "show" or "skip" options should take priority over "timeout", because the latter is more specific. Looks good as is.

[zdykstra]

I'll have to have you re-review this after I write documentation for these new options / new option names.