Fix to not print unnecessary comma.

When `n2->car` is 0 and `n2->cdd` is not 0,
unnecessary comma was printed before first local variables.
add a few const qualifier
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
Merge branch 'master' of github.com:mruby/mruby
Document left mrbconfs.
Quote default value.
Fix typo in doc/api/mruby.h.md
Sacrifice some micro-optimization to remove duplicate code
Merge pull request #2426 from take-cheeze/mrbconf

Document rest mrbconfs.
Merge pull request #2427 from suzukaze/fix-typo

Fix typo in doc/api/mruby.h.md
Merge branch 'master' of github.com:mruby/mruby
reduce mrb_funcall in exception allocation
initialize nomem_err->mesg->c before exposing to Ruby world; close #2428
Fix crash in method call in `ObjectSpace.each_object` block.
* Filter `MRB_TT_ENV` and `MRB_TT_ICLASS`.
* Set `mrb->string_class` in `mrb_init_exception` instead.
ObjectSpace.each_object should filter out internal (i.e. obj->c == NULL) objects; ref #2429
remove String class initialization in error.c; ref #2429
git push origin masterMerge branch 'take-cheeze-fix_each_obj_crash'
Merge pull request #2430 from SHyx0rmZ/remove-duplicate

Sacrifice some micro-optimization to remove duplicate code