MAC_OS_X_VERSION_MAX_ALLOWED reflects the currently running OS X version
more closely than MAC_OS_X_VERSION_MIN_REQUIRED, given it's defined as
max(current_version, MAC_OS_X_VERSION_MIN_REQUIRED).
Additionally, we should check if MAC_OS_X_VERSION_10_9 is actually
defined, otherwise the whole macro logic breaks apart.
Use the (deprecated) Gestalt functionality for fetching the fine-grained
OS X version number on 10.9 and lower.
The newer NSOperatingSystemVersion structure is only available on
10.10+.
This does remove the net* commands from the
Unix version that may return at a later date
with OSX and Windows support.
This commit also makes numerious other changes
such as code cleanup, reformatting, etc.
Closes#829