This patch cleans up the ABI mess, I created at some point.

The initial version of _gfortran_caf_register didn't handle stat/errmsg 
as one could leave it to the front end: The the returned memory is NULL, 
it's an error. However, as Nick pointed out, for stat= one can also 
return STAT_STOPPED_IMAGE. In order to handle this, one needs an 
additional argument.

That's what was done - albeit incompletely: The documentation was 
updated, cf. http://gcc.gnu.org/wiki/CoarrayLib#Registering_coarrays, as 
was the front end (cf. function declaration and call in trans-decl.c); 
however, the library itself (single.c and mpi.c) was not accepting the 
new arguments.

The attached patch solves this: It updates the just (by Daniel) added 
trans.c call and implements the new arguments in the library.

TODO: In trans.c (for the ALLOCATE statement), I currently pass NULL 
pointers for stat and errormsg argument. Hence, the ABI is fixed, but 
the error diagnostic is not yet standard conform. However, I think one 
can defer this to another patch. I added a note in my BUG file to make 
sure it won't get forgotten. Cf. 
http://users.physik.fu-berlin.de/~tburnus/coarray/BUGS.txt

Build and regtested on x86-64-linux.
OK for the trunk?

(Daniel Carrera, I would be happy if you could also have a look at the 
patch.)

Tobias