fmsg: implement suspend in writer

This removes the requirement to call fmsg.Exit on every exit path, and enables direct use of the "log" package. However, fmsg.BeforeExit is still encouraged when possible to catch exit on suspended output.

Signed-off-by: Ophestra <cat@gensokyo.uk>

cmd/fpkg/paths.go

@@ -1,6 +1,7 @@
 package main
 
 import (
+	"log"
 	"os"
 	"os/exec"
 	"path"
@@ -25,8 +26,8 @@ func init() {
 
 func lookPath(file string) string {
 	if p, err := exec.LookPath(file); err != nil {
-		fmsg.Fatalf("%s: command not found", file)
-		panic("unreachable")
+		log.Fatalf("%s: command not found", file)
+		return ""
 	} else {
 		return p
 	}
@@ -35,15 +36,14 @@ func lookPath(file string) string {
 var beforeRunFail = new(atomic.Pointer[func()])
 
 func mustRun(name string, arg ...string) {
-	fmsg.VPrintf("spawning process: %q %q", name, arg)
+	fmsg.Verbosef("spawning process: %q %q", name, arg)
 	cmd := exec.Command(name, arg...)
 	cmd.Stdin, cmd.Stdout, cmd.Stderr = os.Stdin, os.Stdout, os.Stderr
 	if err := cmd.Run(); err != nil {
 		if f := beforeRunFail.Swap(nil); f != nil {
 			(*f)()
 		}
-		fmsg.Fatalf("%s: %v", name, err)
-		panic("unreachable")
+		log.Fatalf("%s: %v", name, err)
 	}
 }