Minor tweaks to delimiter handling in read.c

Minor tweaks to delimiter handling in read.c

* libguile/read.c (CHAR_IS_R5RS_DELIMITER, CHAR_IS_DELIMITER): Move the
  '[' and ']' delimiters from CHAR_IS_R5RS_DELIMITER to
  CHAR_IS_DELIMITER.  Parenthesize all references to the macro
  parameter.  Don't check the global square-brackets read option until
  after we know the character is '[' or ']'.
  (scm_read_sexp): Don't check the global square-brackets read option
  until after we know the character is ']'.