Implement check_is_symlink

Provide a way to test a file is a symlink without worrying about the target
it's pointed to.
Use -L to test for a symlink

The BSD manpage suggests -L is more portable than -h, although POSIX treats
each equally, but you never know.
Add test for check_is_symlink
Merge pull request #267 from Vanders/file-symlink

Implement check_is_symlink