This patch brings Kernel::RangeAllocator to UserspaceEmulator in a
slightly simplified form.
It supports the basic three allocation types needed by virt$mmap():
allocate_anywhere, allocate_specific, and allocate_randomized.
Porting virt$mmap() and virt$munmap() to use the allocator makes
UE work correctly once again. :^)
All users of this mechanism have been switched to anonymous files and
passing file descriptors with sendfd()/recvfd().
Shbufs got us where we are today, but it's time we say good-bye to them
and welcome a much more idiomatic replacement. :^)
