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>

helper/stub.go

@@ -14,7 +14,7 @@ import (
 
 	"git.gensokyo.uk/security/fortify/helper/bwrap"
 	"git.gensokyo.uk/security/fortify/helper/proc"
-	"git.gensokyo.uk/security/fortify/internal/fmsg"
+	"git.gensokyo.uk/security/fortify/internal"
 )
 
 // InternalChildStub is an internal function but exported because it is cross-package;
@@ -40,7 +40,7 @@ func InternalChildStub() {
 		genericStub(flagRestoreFiles(4, ap, sp))
 	}
 
-	fmsg.Exit(0)
+	internal.Exit(0)
 }
 
 // InternalReplaceExecCommand is an internal function but exported because it is cross-package;