harden and platformize FIFO handling

Reject a FIFO path that points to an existing non-FIFO object.
Previously we removed whatever was at that path before creating the
pipe, so pointing -fifo at a regular file silently deleted it. An
existing FIFO is reused and only a missing path is created.

Move the implementation into fifo_unix.go and fifo_windows.go.
This lets the Unix build use syscall.Mkfifo directly, which is not
available on Windows.

Prevent the reader from endlessly spinning on a closed FIFO after an
error.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

main.go

@@ -3,17 +3,14 @@ package main
 import _ "net/http/pprof"
 import (
 	"al.essio.dev/pkg/shellescape"
-	"bufio"
 	"crypto/tls"
 	"flag"
 	"fmt"
-	"io"
 	"log"
 	"net/http"
 	"os"
 	"os/exec"
 	"strings"
-	"syscall"
 	"time"
 
 	barnlog "git.stormux.org/storm/barnard/log"
@@ -75,32 +72,6 @@ func setup_notify_runner(notify_command string) chan []string {
 	return t
 }
 
-func setup_fifo(fn string) (chan string, error) {
-	t := make(chan string)
-	if fn == "" {
-		return t, nil
-	}
-	os.Remove(fn)
-	err := syscall.Mkfifo(fn, 0600)
-	if err != nil {
-		return t, err
-	}
-	file, err := os.OpenFile(fn, os.O_RDWR, os.ModeNamedPipe)
-	if err != nil {
-		return t, err
-	}
-	go func(fh io.Reader, out chan string) {
-		reader := bufio.NewReader(fh)
-		for {
-			line, err := reader.ReadBytes('\n')
-			if err == nil {
-				out <- strings.TrimSpace(string(line))
-			}
-		}
-	}(file, t)
-	return t, nil
-}
-
 func main() {
 	// Command line flags
 	server := flag.String("server", "localhost:64738", "the server to connect to")