Implement --no-max-error-line-width, as requested in Mantis #308.

compiler/options.m:
   Change the type of the max_error_line_width option from int to maybe_int.
   Change its documentation accordingly.

doc/user_guide.texi:
   Change the option's documentation here as well.

compiler/globals.m:
   Since this is the compiler's first maybe_int option, add a predicate
   to look up such options.

compiler/error_util.m:
   If the user specifies --no-max-error-line-width, do not try to pack the
   words of an error paragraph into lines; just return all the words on one
   line.

   Factor out some common code in the group_words predicate, and give that
   predicate a more accurate name.

tests/invalid/any_to_ground_in_ite_cond_nomax.{m,err_exp}:
   Add this copy of the existing any_to_ground_in_ite_cond test case.

tests/invalid/Mercury.options::
   Specify --no-max-error-line-width for the new test case.

tests/invalid/Mmakefile:
   Enable the new test.