Returning errno as a second value

[Daniel Herring]

Hi Felix,

Errno is a particularly tricky beast.  It is a thread-local "global" that 
is modified by a large number of system calls and library functions. 
Code must be careful to check it before another such call is made.  I 
could easily imagine a stop-the-world GC accidentally modifying errno if 
errno is not checked "atomically" with the function call.

Also errno is very lightweight.  An extra foreign call to check it could 
easily dwarf the cost of the check.  Errno is a "hidden" return parameter, 
much like the second position in a multiple-value return.

Thus the Allegro semantics of returning errno as a second value seem 
reasonable to me.

As you suggested, CFFI could provide a default errno implementation for 
CLs that have different native semantics.

- Daniel