Simplify the rescan marker

The rescan marker (used to delimit tokens rescanned via \scantokens) simply
needs to be made of twice the same character with distinct catcodes.  Instead
of using `@' with odd catcodes, simply use `:' (letter) and `:' (other).


git-svn-id: http://www.latex-project.org/svnroot/experimental/trunk@5068 de43f980-851b-0410-b2f7-c40aca1f87e0