collectdctl command hangs on AIX and returns error 0 on Solaris.

Summary:
Problem: collectdctl command hangs on AIX and returns error 0 on Solaris.

Root cause - client (collectdctl) and server (collectd daemon) are using fprintf to communicate using Unix domain socket, Since fprintf buffers, command sent by client did not reach server, since client does not get the response, it closes the socket which forces the client to flush the command, now server receives the command, when responding, it gets socket error because the client already closed the socket.

Solution: Added flush after all fprintf calls.

Test Plan: Tested collectdctl on AIX and SunOS for listval, getval commands
instead of just appending the `fflush` call in the macro, it'll be safer to wrap the whole thing in a do/while:

```c
#define print_to_socket(fh, ...) \
  do { \
    if (fprintf (fh, __VA_ARGS__) < 0) { \
     char errbuf[1024]; \ 
     WARNING ("handle_flush: failed to write to socket #%i: %s", \
     fileno (fh), sstrerror (errno, errbuf, sizeof (errbuf))); \
     return -1; \
  } \
  fflush(fh) \
} while (0)   
    

```