bug#18592: FFI should have portable access to ‘errno’

[guile@discard.email]

I'd like to point out that when calling a function that might set errno in C,
it's usually good practice to set errno to zero immediately before the call,
so as to be sure that any subsequent non-zero errno value was actually
a consequence of the function call:
 
errno = 0; /* clear errno before the function call */
rtnval = my_func(...);
if (errno != 0) {...}
 
This should probably be taken into account if you're extending
the FFI to provide access to errno.