Now that the SystemMonitor queries which open files can be read and written to,
having can_read()/can_write() unconditionally call ASSERT_NOT_REACHED() leads
to system crashes when inspecting the WindowServer.
Instead, just return true from can_read()/can_write() (indicating that the
read()/write() syscalls should not block) and return -EINVAL when trying to
actually read from or write to these devices.
This implements a very basic VGA device using the information provided
to us by the bootloader in the multiboot header. This allows Serenity to
boot to the desktop on basically any halfway modern system.
