sub/main subprofile should not be requested directly
as documented in its README but rather via use/repo/main;
let's fix this discrepancy and check that no regressions
come hurling down.

This could harm the debug...

Document the one-way nature of bootloader type switching.

Applying set() and only set() to a GLOBAL_* is safe
but still a potentially confusing example; so let's
just do it right (and warn unsuspecting folks too).

These were broken since 5ff375aa10bee7937db457d3df3115f938bb6c77
due to no bootloader being tossed anymore by the very
installer-distro-altlinux-generic (which is the default)

NB: needs installer-distro-altlinux-generic = 6.0-alt2
    as it doesn't require alterator-grub indefinitely