Consider using an external argument parser

[oSoMoN]

Argument parsing in the diff command (and in the WIP cmp command) is implemented manually. The rationale for not using a general-purpose parser like clap is that it does not offer the flexibility we need to replicate the GNU diffutils arguments. We'd lose in compatibility what we would gain in code simplicity.

While researching how it's done in uutils/coreutils, I came across uutils/coreutils#4254, which looks especially relevant. Perhaps we should consider giving uutils-args a test run to understand if it fits the needs of the diffutils applications, and provide constructive feedback and contributions in case it doesn't.

There are currently only two CLI applications (out of a total of 4) to migrate, so the advantage over uutils/coreutils is that the effort would be rather limited.