Default conveys more meaning and indicates this plan will become
the default.
Keep the legacy-only helpers together: they are no longer needed
elsewhere or are handled in a simpler way. NANO_MD_BACKING and
NANO_IMAGE_MBRONLY are also moved in, since legacy.sh is the only
plan that uses them.
The defaults.sh file is not meant to be executed directly.
Bump the default NANO_RAM_TMPVARSIZE since pkgbase requires
significantly more space in /var/db/pkg. Leave the legacy plan
default unchanged.