Resident executable support stage 1/4: Add kernel bits and syscall support

Resident executable support stage 1/4: Add kernel bits and syscall support
for in-kernel caching of vmspace structures.  The main purpose of this
feature is to make it possible to run dynamically linked programs as fast
as if they were statically linked, by vmspace_fork()ing their vmspace and
saving the copy in the kernel, then using that whenever the program is
exec'd.