Add sanity checking for prefix-length
Add sanity checking for prefix-length

This adds some pretty thorough sanity checking for the prefix-length
input field.

Fixes #226

plajjan/NIPAP,ettrig/NIPAP,SpriteLink/NIPAP,bbaja42/NIPAP,garberg/NIPAP,plajjan/NIPAP,fredsod/NIPAP,SoundGoof/NIPAP,bbaja42/NIPAP,bbaja42/NIPAP,ettrig/NIPAP,SpriteLink/NIPAP,fredsod/NIPAP,fredsod/NIPAP,ettrig/NIPAP,bbaja42/NIPAP,garberg/NIPAP,SpriteLink/NIPAP,SoundGoof/NIPAP,plajjan/NIPAP,fredsod/NIPAP,SpriteLink/NIPAP,plajjan/NIPAP,plajjan/NIPAP,garberg/NIPAP,garberg/NIPAP,SoundGoof/NIPAP,SoundGoof/NIPAP,SoundGoof/NIPAP,plajjan/NIPAP,ettrig/NIPAP,SpriteLink/NIPAP,SpriteLink/NIPAP,fredsod/NIPAP,fredsod/NIPAP,bbaja42/NIPAP,bbaja42/NIPAP,ettrig/NIPAP,garberg/NIPAP,ettrig/NIPAP,SoundGoof/NIPAP,garberg/NIPAP