Hi,

when killing shepherd i.e `pkill -9 shepherd` it left behind
`default-socket-file` and when restarted whithout remove the socket like
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rm /var/run/user/1000/shepherd/socket
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it throws a error:
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3 (primitive-load "/home/gfleury/prod/shepherd/./shepherd")
In shepherd.scm:
    56:14  2 (main . _)
     49:6  1 (open-server-socket _)
In unknown file:
           0 (bind #<input-output: socket 16> #(1 "/run/user/1000?") #)

ERROR: In procedure bind:
In procedure bind: Address already in use
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something like this patch can fix it.