Add the initial sysctl implementation code.

(It's not yet linked into the build or in the exported symbols list.)
Add a local copy of the libnv from -HEAD.

This is used by the upcoming sysctl code to serialise data in and out.
Add a local version of memstat, to pull allocator data out of libuinet.
Add local copies of sysctl/vmstat, hacked up to (mostly) work within
libuinet.
Implement rmlock and sxlock directly using the uhi rwlock API instead
of reusing the rwlock implementation, as reusing the rwlock
implementation gets in the way of WITNESS support.
Merge pull request #19 from erikarn/adrian_sysctl_prep

This is just the prep work - it doesn't include the build framework changes to build it, or the modifications to programs to call the sysctl setup path.