[RFC] Replace vim_strncpy with strlcpy

> Actually I was thinking about what this all implies:
>
>   ret == size - 1: the caller obviously knew what was going on. This should probably be a memcpy/stracpy.
>   ret >> size: truncation, why?!
>   ret << size: probably quite alright.

Yeah, the first could be a `DLOG` suggesting `memcpy/stracpy`. The second case a `WLOG` and the third a `DLOG` as we (I?) are somehow worried that the codebase relies on `strncpy`s zero-filling behavior.

By separating the cases like this I can turn off debug level messages in the `nvim` that I use for everything (yep, I install `nvim`'s `Debug` build on my system).