loader: Rework kernel menu section

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.

Differential Revision: https://reviews.freebsd.org/D48354
Reviewed by: imp, tsoome
Sponsored by: Beckhoff Automation GmbH & Co. KG