rio.c fdset target: tolerate (and report) a subset of FDs in error.

Fdset target is used when we want to write an RDB file directly to
slave's sockets. In this setup as long as there is a single slave that
is still receiving our payload, we want to continue sennding instead of
aborting. However rio calls should abort of no FD is ok.

Also we want the errors reported so that we can signal the parent who is
ok and who is broken, so there is a new set integers with the state of
each fd. Zero is ok, non-zero is the errno of the failure, if avaialble,
or a generic EIO.

src/rio.h

@@ -74,7 +74,8 @@ struct _rio {
         } file;
         /* Multiple FDs target (used to write to N sockets). */
         struct {
-            int *fds;
+            int *fds;       /* File descriptors. */
+            int *state;     /* Error state of each fd. 0 (if ok) or errno. */
             int numfds;
             off_t pos;
         } fdset;