Also, unterminated strings are highlighted with the "error" color now.

* The colon modifier can now occur 2 times.
  Specifying `@` more than once or `:` more than twice is an error now.
* Commands do not check for excess colon modifiers - almost every command would have
  to check it. Instead, a double colon will simply behave like a single colon on most
  commands.
* All search commands inherit the anchored semantics, but it's not very useful in some combinations
  like -::S, ::N or ::FK.
  That's why the `::` variants are not documented everywhere.
* The lexer.checkheader macro could be simplified and should also be faster now,
  speeding up startup.
  Eventually this macro can be made superfluous, e.g. by using 1:FB or 0,1^Q::S.

* The device tree lexer reuses CPP and has certain limitations.
  For once it does not recognize /keywords/ and secondly it confuses
  properties beginning with # as preprocessor statements.