Start[all] fixes: parameter check and return code

[devreal]

This PR fixes two three glitches in MPI_Start and MPI_Startall:

1. MPI_Startall wouldn't accept partitioned requests if parameter checking was enabled.
2. Check for the request state to be OMPI_REQUEST_INACTIVE in MPI_Start to mirror the check in MPI_Startall.
3. Check return value of request start function to prevent error codes from leaking.

[devreal]

Found another one and added it to this PR: MPI_Start didn't check for the return value of the start callback.