loader: Rework kernel menu section

[manu]

Summary

With pkgbase we can have long kernel name, so create a new section
for the kernel name.
Do not show the "default" text, we already show the "1 of X" part at
the end of the line and the default kernel is always number 1 so it's a bit
redundant.

Sponsored by: Beckhoff Automation GmbH & Co. KG

Test Plan

Before: https://people.freebsd.org/~manu/IMG_20250107_095722.jpg
After: https://people.freebsd.org/~manu/IMG_20250107_095848.jpg

[imp]

I think this is sane...

stand/lua/menu.lua

350

thanks for preserving this.