Rework MANIFEST generation:

• Use the built-in printf in place of echo to eliminate the need for inline tabs.
• Flip ${foo}_default to default_${foo} for consistency with the desc_${foo} variables.
• Add debugging distributions, which are off by default, and descriptions.
• Replace '.' and '-' in distribution names with '_', to work around 'eval' emitting errors.
• Add support for selecting a non-GENERIC kernel, and provide the KERNCONF name in the description.

Sponsored by: The FreeBSD Foundation