unix: return exec errors from uv_spawn, not async

If spawning a process fails due to an exec() failure (but it succeeded
in forking), then this should be considered a spawn failure instead of
an asynchronous termination of the process. This allows to check for
common exec() failure conditions such as a bad path quickly instead of
having to rely on keeping track of the async callback.

Additionally, the meaning of the two fields returned in the callback are
now exactly what they advertise to be. The process exit argument is not
one of two values depending on what happened to the child.

Fixes #978.