ui/termstatus: extract background handling code

internal/terminal/tcgetpgrp_linux.go

@@ -0,0 +1,12 @@
+package terminal
+
+import "golang.org/x/sys/unix"
+
+func Tcgetpgrp(ttyfd int) (int, error) {
+	// We need to use IoctlGetUint32 here, because pid_t is 32-bit even on
+	// 64-bit Linux. IoctlGetInt doesn't work on big-endian platforms:
+	// https://github.com/golang/go/issues/45585
+	// https://github.com/golang/go/issues/60429
+	pid, err := unix.IoctlGetUint32(ttyfd, unix.TIOCGPGRP)
+	return int(pid), err
+}