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