* Fix illumos managed build
* Fix CA1823 (unused private field) in `NetworkChange` partial for
  `UnknownUnix`.
* Use official casing `illumos` in MSBuild property names (as done for
  iOS).
* Fix Solaris version in test with SDK's PlatformDetection.
  * only major version is needed.

* Implement Enviornment.WorkingSet for SunOS
Difference between Linux and SunOS procfs is that files in latter
contain binary data, so we need `read(2)` and cast into corresponding
struct. Redeclaring system structs in managed code is not ideal, as
they do change across the major versions of OS, which inevitably
requires recompilation of binaries and replicating them in C# as is
means additional/unnecessary maintenance of code.

* Address CR feedback