Move zfs_acl_alloc from sys/zfs_acl.h to zfs_acl.c.

It is only used in zfs_acl.c, and declared static, so it was probably moved
wholesale with others.
Typedef minor_t to int in zfs_context.h.

This is the typical definition for minor_t elsewhere.
Consequently, reactivate dsl_register_onexit_hold_cleanup.
Enable debug builds and separate strict compiles.

Use DEBUG rather than NDEBUG, because that's what's used in illumos.
Re-add znode_stalker support, as included in older maczfs.
Make debug builds work with the zfs module itself, not just userspace.
minor_t was defined in sys/zfs_context.h.
ifdef UIO_XUIO xuio code, which is not needed.