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>

internal/system/mkdir.go

@@ -40,7 +40,7 @@ func (m *Mkdir) Type() Enablement {
 }
 
 func (m *Mkdir) apply(_ *I) error {
-	fmsg.VPrintln("ensuring directory", m)
+	fmsg.Verbose("ensuring directory", m)
 
 	// create directory
 	err := os.Mkdir(m.path, m.perm)
@@ -61,11 +61,11 @@ func (m *Mkdir) revert(_ *I, ec *Criteria) error {
 	}
 
 	if ec.hasType(m) {
-		fmsg.VPrintln("destroying ephemeral directory", m)
+		fmsg.Verbose("destroying ephemeral directory", m)
 		return fmsg.WrapErrorSuffix(os.Remove(m.path),
 			fmt.Sprintf("cannot remove ephemeral directory %q:", m.path))
 	} else {
-		fmsg.VPrintln("skipping ephemeral directory", m)
+		fmsg.Verbose("skipping ephemeral directory", m)
 		return nil
 	}
 }