Correct includes for unlockiter.c

Some platforms (e.g., Solaris) need a declaration of memset() for the
FD_ZERO() macro to work, contrary to POSIX standards.  Add an
inclusion of <string.h> to accommodate them.  Also add <sys/time.h>,
possibly needed by some older platforms, and remove a spurious
inclusion of <sys/socket.h>.

ticket: 7977