Add stpeprintf()

Add stpeprintf()

[v]stpeprintf() are similar to [v]snprintf(3), but they allow chaining.
[v]snprintf(3) are very dangerous for catenating strings, since the
obvious ways to do it invoke Undefined Behavior, and the ways that avoid
UB are very error-prone.

Cc: Iker Pedrosa <ipedrosa@redhat.com>
Signed-off-by: Alejandro Colomar <alx@kernel.org>