Incorrect return type

Created by: tdbgamer

https://github.com/redox-os/relibc/blob/d75535974ae37e7d698c01c46584705b790e0bc9/src/string/src/lib.rs#L277

I believe this function is supposed to return size_t. Actually it looks like all the functions in that file that return c_ulong should return size_t. I modified the type and the tests passed fine for me. Should I open a PR?