sysutils/grub2: Fix grub-mkconfig

Currently grub-mkconfig creates a bogus grub.cfg with lines like

tinsmod part_gpt
tinsmod ufs2

grub-mkconfig and helpers use sed expressions like "s/^/\t/" or
similar to indent grub.cfg with tabs which only really work as
intended with gsed at the moment and currently just prefixes lines
with a 't'. Switch grub-mkconfig and other scripts over to gsed.

help2man calls some scripts at build time, so add gsed as a build
dependency too.

PR: 228062
Reported by: vas@mpeks.tomsk.su