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.

The features might get copy-pasted (or even copied-and-pruned)
when initialized; there's an unneccessary duplication of the
function name in the line adding it to FEATURES list, thus
prone to being forgotten and causing some havoc later on.

It was wrong in the first place but tackling this with some
double-colon rules ran into terminality issues, and further
tortures were considered unneccessary.

The current solution isn't perfect (no completely transparent
function name registration upon corresponding target being called)
but at least it is an improvement...

ToC:
- introduced doc/
- features.in/00example/
- READMEs and TODO for more
- code comments clarified

Some unrelated minor fixes bundled either.

- get rid of $dest
- work on features.in/syslinux
- README++