vkernel - Module loading for vkernel64

vkernel - Module loading for vkernel64

* Fix a missing vm_object_drop() in the kldload error path.

* Load modules using sbrk()'d memory (for now).

* Hack rtld to accept a new environment variable, LD_SHAREDLIB_BASE,
  which forces shared libraries to be mmap()'d at the specified address.

* Hack the vkernel64 binary to re-exec itself in order to set
  LD_SHAREDLIB_BASE to low memory.  This forces shared libraries into
  low memory which the KLD module loader can relocate against, instead
  of high memory which it can't.

* test/vkernel/Makefile now builds and installs modules by default

With-fixes-from: tuxillo