functions/fs: add infer_drive, reusable copy of stage-pool/util-hier-infer-drive@.sh.
stage-list-pool/6-post-configure/0-post-install/3-dest-install-grub@.sh: use infer_drive directly.

This is more robust (less inter-stage-list dependencies).
stage-list-pool/2-prepare-dest: remove 2-util-hier-infer-drive@.src.sh (unused), and remove the stage itself.
stage-pool/dest-conf-user@.sh: issue a userdel before adding the user.

For robustness.
stage-pool/dest-conf-user-passwd@.sh: fix stage (passwd does not support password on cmdline).
stage-list-pool/9-user: fix invocation of dest-conf-user* stages.
stage-data/dest-install-configs@.sh/...: modify mkinitcpio.conf to workaround a deficiency in our setconf implementation.
stage-pool/dest-conf-mkinitcpio-modules@.sh: use list_set_file to avoid duplicate entries.
stage-pool/dest-conf-grub-cmdline@.sh: fix parameter key to set (no _DEFAULT).