Typographic fixes, spacing after \opt

* occurrences of "\opt \terminal{" apparently always require "\opt{}" to not eat
  the intervening space (despite the \xspace in the definition of \opt)

* cleaned up two occurrences of "\opt\ " (in

    noptr-abstract-pack-declarator \terminal{[} constant-expression\opt\ \terminal{]} attribute-specifier-seq\opt\br

  in declarators.tex and grammar.tex) to consistenly use "\opt{}" instead

* cleaned up two occurrences of "\opt{}" (in

    pp-tokens\opt{} new-line

  in grammar.tex and preprocessor.tex) that did not need the "{}"