New std/basic library for very basic definitions
  - lnfix, lifix (formerly in xdoc/top for no good reason)
  - maybe-natp, maybe-posp, maybe-integerp, maybe-stringp (formerly in vl
    and other internal Centaur books)
  - true, false (formerly in internal Centaur books)

Fixes to VL and other miscellaneous libraries to accomodate std/basic

XDOC updates:
  - Add linkcheck.html page with external links and image URLs in one place,
    for use with link-checking programs. (fixes issue 118)
  - Fix up acl2-doc.lisp topics to <icon> for the tiny warning sign images.
  - Add (undocumented) icon tag for inline images.  (I had broken the tours
    badly by centering images, this is basically a workaround for that.)
  - Horrible (undocumented) hacks to get colors working in a reasonable,
    consistent way across @(see ...), @(tsee ...), @('...'), and @({...}).
  - Slight fixes to topic origin printing

Defrstobj:
  - Added basic documentation
  - Moved projects/defrstobj to projects/legacy-defrstobj
  - Documented projects/legacy-defrstobj
  - Added legacy-defrstobj make target

Leftist-trees:
  - Add copyright headers to files

git-svn-id: http://acl2-books.googlecode.com/svn/trunk@2295 303c5b2c-9740-0410-b014-fd8f9db7be32