sysutils/spiped: Clean up UNIX sockets

When a TCP socket is closed, it becomes possible to create a new
socket listening on the same address; the behaviour of UNIX (aka
"local") sockets is different, in that an inode remains even after
it is closed, and blocks the creation of a new socket with the same
address.

When spiped is launched with a UNIX socket as its source address,
delete any existing socket with that address first. This makes it
possible to "service spiped restart" when UNIX sockets are used.

Deleting the socket when stopping spiped would also work for the
case of restarting the daemon, but not for the case of starting the
daemon after an unclean system shutdown; so deleting only prior to
starting the daemon seemed like the better option.

PR: 295432
Reported by: feld