Handle TAP-Win32 immediate reads correctly.

The handling of TAP-Win32 virtual network device reads that complete
immediately (ReadFile() returns TRUE) is incorrect - instead of
starting a new read, tinc will continue listening for the overlapped
read completion event which will never fire. As a result, tinc stops
receiving packets on the interface.
Fix exit code of "tinc get".

Successfully getting an existing variable ("tinc get name") should
not result in an error exitcode (1) from the tinc command.

This changes the result of test/commandline.test from FAIL to PASS.
commandline.test: Adding test that fetching non-existing config setting really fails.