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
Implement Proc#parameters
Add documentation of `mrb_value` config.
Add notes about restriction of `DISABLE_STDIO`.
Merge pull request #2425 from take-cheeze/mrbconf

Update mrbconf.
Merge pull request #2423 from ksss/proc-parameters

Implement Proc#parameters
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