lib/printf: Implement N2680.

lib/printf: Implement N2680.

This adds specific width length modifiers in the form of wN and wfN
(where N is 8, 16, 32, or 64) which allow printing intN_t and
int_fastN_t without resorting to casts or PRI macros.

Reviewed by:	imp, emaste
Differential Revision:	https://reviews.freebsd.org/D41725

FRR changes only include printf(), scanf/strtol are not locally
implemented in FRR.  Also added "(void) 0" to empty "else ..." to
avoid a compiler warning.

Signed-off-by: David Lamparter <equinox@opensourcerouting.org>
(cherry picked from FreeBSD commit bce0bef3c6abab92c7ac8cc23b7cc632a382721e)