backend,termstatus: Unify foreground/background detection

PR #5358 reintroduced a version of the TIOCGPGRP ioctl call that works
on all Unix platforms except Linux, due to a bug/inconsistency in
x/sys/unix. This commit fixes that by introducing termstatus.Tcgetpgrp.

It also introduces termstatus.Getpgrp and termstatus.Tcsetpgrp to deal
with the different signature of unix.Getpgrp in Solaris vs. all other
Unix platforms and an int-overflowing constant on AIX, so that some
AIX/Solaris-specific code can be removed elsewhere and
foreground/background detection is done the same everywhere except on
Windows.

internal/ui/termstatus/background_unix_test.go

@@ -0,0 +1,22 @@
+//go:build unix
+
+package termstatus
+
+import (
+	"os"
+	"testing"
+
+	rtest "github.com/restic/restic/internal/test"
+)
+
+func TestIsProcessBackground(t *testing.T) {
+	tty, err := os.Open("/dev/tty")
+	if err != nil {
+		t.Skipf("can't open terminal: %v", err)
+	}
+
+	_, err = isProcessBackground(int(tty.Fd()))
+	rtest.OK(t, err)
+
+	_ = tty.Close()
+}