this lets us *dis*able filesystem explicitly, as is required by e.g. the
zfs-less installer images. currently that specifically is only easily
possible by adding an overlay that stubs out `zfs`, with the obvious
side-effect of also removing tooling that could run without the kernel
module loaded.
Currently, ecryptfs support is coupled to `security.pam.enableEcryptfs`, but one
might want to use ecryptfs without enabling the PAM functionality. This commit
splits it out into a `boot.supportedFilesystems` switch.