lib/atoi/strtoi.[ch]: strtoi_(), strtou_(): Add functions

lib/atoi/strtoi.[ch]: strtoi_(), strtou_(): Add functions

These functions are identical to strtoi(3bsd) and strtou(3bsd), except
for one important thing: if both ERANGE and ENOTSUP conditions happen,
the BSD functions report ENOTSUP, which is bogus; our strtoi_() and
strtou_() report ERANGE.

Link: <https://lists.sr.ht/~hallyn/shadow/%3CZZoQDms6Sv6e5SPE%40debian%3E>
Link: <https://gnats.netbsd.org/cgi-bin/query-pr-single.pl?number=57828>
Cc: Thorsten Glaser <tg@mirbsd.de>
Cc: christos <christos@netbsd.org>
Cc: roy <roy@netbsd.org>
Cc: Guillem Jover <guillem@hadrons.org>
Signed-off-by: Alejandro Colomar <alx@kernel.org>