generators: rename add_{root,usr}_mount to add_{sysroot,sysroot_usr}_mount

This makes it obvious that those functions are only usable in the
initramfs.

Also, add a warning when noauto, nofail, or automount is used for the
root fs, instead of silently ignoring. Using those options would be a
sign of significant misconfiguration, and if we bother to check for
them, than let's go all the way and complain.

Other various small cleanups and reformattings elsewhere.