Wrong behavior of File.extname when the filename starts or ends with a period

Behavior of File.extname is incompatible with CRuby.

In CRuby

```
$ irb
irb(main):001:0> File.extname('.a.rb')
=> ".rb"
irb(main):002:0> File.extname('foo.')
=> "."

```
In mruby
```
$ mirb
mirb - Embeddable Interactive Ruby Shell

> File.extname('.a.rb')
 => ""
> File.extname('foo.')
 => ".foo"

```
