Change rman_manage_region() to actually honor the rm_start and rm_end
constraints on the rman and reject attempts to manage a region that is out
of range.

• Fix various places that set rm_end incorrectly (to ~0 or ~0u instead of ~0ul).
• To preserve existing behavior, change rman_init() to set rm_start and rm_end to allow managing the full range (0 to ~0ul) if they are not set by the caller when rman_init() is called.