nbd-server fails silently with unixsock existing

[ari-s]

when configured with unixsock = /run/nbd-test.sock and the location /run/nbd-test.sock exists, nbd-server silently fails. Only when non-forking

# [ -e /run/nbd-test.sock ] && echo /run/nbd-test.sock existing 
/run/nbd-test.sock existing
# nbd-server ; echo $? ; ps aux|grep nbd
0
root       20820  0.0  0.0   6180   552 pts/0    S+   11:59   0:00 grep nbd

# nbd-server -d; echo $?
1

systemctl start nbd-server "succeeds" even though nbd-server itself isn't actually started

This is related to #122