cryptsetup password and parallel boot

Since parallel boot happened, during boot the cryptsetup password prompt in my system gets flooded with other boot messages.

I fixed it, as suggested in #764555, installing plymouth, then editing /etc/default/grub to add splash to GRUB_CMDLINE_LINUX_DEFAULT:


Besides showing pretty pictures (and most importantly, getting them out of my way if I press ESC), plymouth also provides a user prompt that works with parallel boot which sounds like what I needed.