Add support for UNIX socket and named pipe to SELECT INTO OUTFILE 5.6

Allow SELECT INTO OUTFILE/DUMPFILE to be targeted to unix socket or named pipe.
For the case when OUTFILE/DUMPFILE exists and is unix socket or pipe instead of
exit with error, connect to the socket or open the fifo and write output
into it.
Testcase verifies three cases
- regular file exists
- socket exists
- fifo exists