Fix IsProcessBackground on Linux with stdin redirection

The previous implementation assumed that stdin was a terminal.
It now checks the terminal's fd.

internal/ui/termstatus/background.go

@@ -4,6 +4,6 @@ package termstatus
 
 // IsProcessBackground reports whether the current process is running in the
 // background. Not implemented for this platform.
-func IsProcessBackground() bool {
+func IsProcessBackground(uintptr) bool {
 	return false
 }