(you decide if it's fool-proof enough for the case at hand)
`readlink /proc/cygdrive' does look like the best solution.
So, the best way I see is to use it and fall back to the heuristics if it's
"Shelling out" can't be avoided unless you wish to check the registry by hand:
initially (in 2000), there was no dedicated utility to read the prefix at all,
the docs pointed directly to the registry
I'll now check the Cygwin repo for /proc/cygdrive addition time vs. registry
setting elimination time to see if one can avoid the heuristics entirely and
skip from the former directly to the latter.