file_descriptor_sink is missing the definition for its templated open() function.

[Aaron Barany <akb825@…>]

file_descriptor_sink has a templated function to open based on any path type, but the function has no definition, resulting in linker errors if you try to call it. I have attached a patch that implements the function by calling open() with detail::path, the same as how file_descriptor and file_descriptor_source do it.

[Aaron Barany <akb825@…>]

Patch to provide the function definition.