fix spurious EINTR errors from multithreaded set*id, etc.

fix spurious EINTR errors from multithreaded set*id, etc.

commit 78a8ef47c4d92b7680c52a85f80a81e29da86bb9 inadvertently removed
the SA_RESTART flag from the sigaction for the internal signal handler
used by __synccall for broadcasting. as a result, programs which did
not use interrupting signals but which used set*id() in a
multithreaded context could wrongly observe EINTR errors they're not
prepared to handle.