server: reopen standard streams to /dev/null

server: reopen standard streams to /dev/null

For some custom ioengines (see https://github.com/spdk/spdk/issues/1118),
it's not trivial to suppress output to stdout and stderr, so they would
write to some other file descriptor fio had opened - which is bad.

This change ensures that fd's 0, 1, and 2 (stdin, stdout, stderr) are
always valid and can be used without any unintended consequences.

Signed-off-by: Andreas Economides <andreas.economides@nutanix.com>