UsageFAQ: General fixes

UsageFAQ: General fixes

Not fixing everything, just trying to fix some of the more egregiously
incorrect or outdated information.

 - Mention aklog in addition to klog
 - Try to remove references to the AFS-ified "login"
 - Update byte-range lock info
 - Remove the AFS-specific fsck stuff; this is inode-specific and
   getting into inode vs namei is probably too detailed for here. This
   isn't a general difference about AFS
 - Mentioned keyctl show for determining Linux PAGs
 - Say that file size on disk varies depending on backend fs
 - Say that foreign users do actually exist
 - Try to remove references to broken URLs, such as links to transarc
   resources