MFC r311236,r311919:

r311236:

unlink_fifo: don't leak the file descriptors opened with mkfifo and open

MFC fater: 3 days
CID: 978316, 978317

r311919:

Partially revert r311236

There's no sense in trying to close a file descriptor from the negative cases
with unlink_test; it's best to ignore these cases.

The mkfifo case does make sense to keep though.