Adjust method definition place of Process.coerce_rlimit_resource

* process.rb: define -> none
* process18.rb none -> define
* process19.rb define -> define
Fix typo in an exception message
Add specs for Process.(g|s)etrlimit on X19

It coreces String as Symbol on X19.
Fix Process.coerce_rlimit_resource for String argument on X19

* MRI accepts String as resource name on Process.(r|s)etrlimit argument.

When passed resource name.

* Don't use #to_sym. Use #is_a? or #kind_of? for check Symbol.
* Use #to_str for check String.
Merge pull request #2164 from kachick/fix-process_rlimit_accessor_for_string_resource_name

Improve Process.(g|s)etrlimit