When creating an initial default config, we usually probe for a free
TCP port. But when a UNIX socket is specified via the `STGUIADDRESS=`
override or the `--gui-address=unix:///...` command line syntax, parsing
that option will fail during port probing.
The solution is to just skip the port probing when the address is
determined to specify something other than a TCP socket.
### Testing
Start with a fresh home directory each time.
1. Specify a UNIX socket for the GUI (works with this PR):
TMPHOME=$(mktemp -d); ./syncthing --home=$TMPHOME
--gui-address=unix://$TMPHOME/socket
2. Specify no GUI address (probes for a free port if default is taken,
as before):
TMPHOME=$(mktemp -d); ./syncthing --home=$TMPHOME
3. Specify a TCP GUI address (probes whether the given port is taken,
as before):
TMPHOME=$(mktemp -d); ./syncthing --home=$TMPHOME
--gui-address=127.0.0.1:8385
b9c6d3ae09