We change the way application processes bind to the private and shared
memory heaps used in fastsync support. Aside of an obscure and
outdated implementation, using /dev/rtheap for this purpose led to a
couple of nasty issues:

- the requirement for abusing mmap()'s offset argument in the no-MMU
  case, so that different processes can map in distinct private heap
  areas.

- as a consequence of the previous point, the exposure of kernel
  addresses to userland (passed as differenciating "area" tags in the
  mapping offset) creates an issue with mixed 32/64bit ABI models.

To fix this, we introduce built-in, RTDM-based memory device drivers
in the Cobalt kernel (aka "UMM" for user-mapped memory), which expose
mmap() interfaces for binding to the proper private or shared
heap. The drivers are called by lib/cobalt when initializing a new
application process, to perform all the necessary mappings.

In addition, a third built-in driver honors (read-only) requests to
get status information about the system heap.