- Document why usr.bin/vi needs to be built as part of bootstrap-tools
- Do not build usr.bin/vi as part of bootstrap-tools if NO_SHARE is defined
  as share/termcap will be skipped if NO_SHARE is defined; that being said,
  something needs to be done for dealing with make distribute as it will be
  built via make distribute if not done during make buildworld.
- Add -DNO_SHARE to NOFUN to deal with make xdev because of the fact that it
  implements a partial make buildworld/make installworld, but because it
  doesn't define everything, not all install paths exist in expected stages,
  which results in build failures with usr.bin/vi in bootstrap-tools because
  "WORLDTMP" is not fully populated when _xb-bootstrap-tools is run.

  tl;dr: this is enough to just make sure that make xdev isn't broken on this
  branch when MK_VI == no and make delete-old has been run on the build host