[$] Kernel API specification and validation

The kernel project makes a strong promise to its users: the kernel ABI willnot be changed in ways that break user-space code. The occasional failurenotwithstanding, kernel developers do try to live up to that promise. Theyare handicapped by one little problem, though: there is no description ofwhat the kernel ABI is, and no comprehensive way to test whether a givenchange breaks it. The kernel APIspecification framework proposed (in its second revision) by SashaLevin addresses some of those concerns, but the solution is incomplete anddoes not come for free.