Consider renaming "editline" references to "libedit"

There is a slightly unrelated project called editline[^1][^2], which seems to be from Minix. While mrsh refers to *libedit*[^3], which seems to be from NetBSD.
I think I'm not the only one which got a bit confused when packaging mrsh and seeing *editline* being an actual thing.

[^1]: https://troglobit.com/projects/editline/
[^2]: https://repology.org/project/editline-troglobit/versions
[^3]: https://repology.org/project/libedit/information