MAC/do: Better parsing for IDs (strtoui_strict())

[olce]

Summary

This revision is part of a series. Click on the Stack tab below to see the context.
This series has also been squeezed into D47633 to provide an overall view.

Commit message:
Introduce strtoui_strict(), which signals an error on overflow contrary
to the in-kernel strto*() family of functions which have no 'errno' to
set and thus do not allow callers to distinguish a genuine maximum value
on input and overflow.

It is built on top of strtoq() and the 'quad_t' type in order to achieve
this distinction and also to still support negative inputs with the
usual meaning for these functions. See the introduced comments for more
details.

Use strtoui_strict() to read IDs instead of strtol().

[olce]

Replace strtonni with strotui_strict().

This is a followup to the change in D47613 (u_int used for IDs). Took the
occasion to provide a better implementation that supports negative number
specifications (as strtou*() standard functions do).

[olce]

Remove a redundant check that was forgotten in the previous update.