Entirely remove exec_map from the kernel. Use the new vm_fault_object_page()

Entirely remove exec_map from the kernel.  Use the new vm_fault_object_page()
for the data/bss special case in the elf loader, and use the objcache to
cache arguments for exec.

This in turn removes nearly all of the SMP page invalidation IPIs that
occur during fork/exec/exit/wait sequences by virtue of not needing to
map and unmap so much KVM.