MFC: r172623, r175681, r176182, r176184, and r185217

• Document quotachecks use of the passno field.
• Explain how the passno field in /etc/fstab works with fsck and quotacheck in some more detail.
• Explain in more detail how the pass number field works.
• Remove information about block devices; we don't have this sort of separation between character and block devices anymore.
• Enhance the explanation of using filesystem-specific mount options, and add a mount_msdosfs example.