`setopt` in main.c is wrong

[umanwizard]

It's seemingly meant to advance the string in e.g. seed=foo by five characters resulting in foo, but what it actually does is advance the first character by 5 resulting in xeed=foo1.

[jwerle]

hi @umanwizard ! thanks for opening the issue. I am pretty busy these days, but happy to review a PR should you want to investigate this.