refactor logging (#582)

refactor logging (#582)

* refactor logging to

allow the noreturn attribute on fatal() to be used by analysis
tools while still generating messages with an output stream.

* Add our own message handler.

This allows us to use stderr on windows.  The default message handler
routes QDebug messages to the debugger on windows.

* flush stdout before message logger writes to stderr.

* flush stdout before writing to standard error for fatal().

or else messages can appear in the wrong order on Windows.