CMake apparently doesn't do anything with CMAKE_OSX_DEPLOYMENT_TARGET
for swift, which results in a 12+ minimum version. This fixes it
(albeit in a hacky way since the only apple-sanctioned way to properly
set this appears to be "use xcode").
Shame on Apple, as usual.
Adds support for building Lokinet as a system extension, and fixes
various problems in the macos implementation found during development of
the system extension support.