Disable custom signal handling if not on foreground

If pscale is used in scripts with the explicit
PSCALE_ALLOW_NONINTERACTIVE_SHELL env flag, disable the signal changes
we do for making the shell work in interactive mode.

This check also needs to verify that all of stdin, stdout & stderr are a
proper TTY and not just stdout, since if you pipe in only input for
example we still need to disable this since there's no foregrounding.

internal/cmd/shell/shell.go

@@ -55,10 +55,12 @@ second argument:
 
 			database := args[0]
 
+			runForeground := true
 			if !printer.IsTTY || ch.Printer.Format() != printer.Human {
 				if _, exists := os.LookupEnv("PSCALE_ALLOW_NONINTERACTIVE_SHELL"); !exists {
 					return errors.New("pscale shell only works in interactive mode")
 				}
+				runForeground = false
 			}
 
 			mysqlPath, err := cmdutil.MySQLClientPath()
@@ -212,7 +214,7 @@ second argument:
 			}()
 
 			go func() {
-				errCh <- m.Run(ctx, sigc, signals, mysqlArgs...)
+				errCh <- m.Run(ctx, sigc, signals, runForeground, mysqlArgs...)
 			}()
 
 			go func() {
@@ -255,7 +257,7 @@ type mysql struct {
 }
 
 // Run runs the `mysql` client with the given arguments.
-func (m *mysql) Run(ctx context.Context, sigc chan os.Signal, signals []os.Signal, args ...string) error {
+func (m *mysql) Run(ctx context.Context, sigc chan os.Signal, signals []os.Signal, runForeground bool, args ...string) error {
 	c := exec.CommandContext(ctx, m.mysqlPath, args...)
 	if m.dir != "" {
 		c.Dir = m.dir
@@ -271,10 +273,12 @@ func (m *mysql) Run(ctx context.Context, sigc chan os.Signal, signals []os.Signa
 	c.Stderr = os.Stderr
 	c.Stdin = os.Stdin
 
-	c.SysProcAttr = sysProcAttr()
-	cancel := setupSignals(ctx, c, sigc, signals)
-	if cancel != nil {
-		defer cancel()
+	if runForeground {
+		c.SysProcAttr = sysProcAttr()
+		cancel := setupSignals(ctx, c, sigc, signals)
+		if cancel != nil {
+			defer cancel()
+		}
 	}
 
 	return c.Run()

internal/printer/printer.go

@@ -20,7 +20,8 @@ import (
 	"github.com/mattn/go-isatty"
 )
 
-var IsTTY = isatty.IsTerminal(os.Stdout.Fd()) || isatty.IsCygwinTerminal(os.Stdout.Fd())
+var IsTTY = (isatty.IsTerminal(os.Stdout.Fd()) && isatty.IsTerminal(os.Stderr.Fd()) && isatty.IsTerminal(os.Stdin.Fd())) ||
+	(isatty.IsCygwinTerminal(os.Stdout.Fd()) && isatty.IsCygwinTerminal(os.Stderr.Fd()) && isatty.IsCygwinTerminal(os.Stdin.Fd()))
 
 // Format defines the option output format of a resource.
 type Format int