The only useful changes to nice here are the move of usage, which is more stylistically consistent with the rest of the source, and the proper POSIX return value.
I was under the impression that this project includes ARGBEGIN/ARGEND macros specifically to avoid getopt, so I see no reason to change to it; it's more verbose anyway. The nice system call automatically clamps the specified value to a reasonable number, and even within linux, acceptable values have changed enough that including a clamp results in nice being less stable, not more. The change to estrtol could be useful, but it could be better to simply replace the atoi call with it, rather than introduce an extra variable just to delay the exit until the end of the input loop.
