lib/string/: Redesign stpecpy() and stpeprintf()

lib/string/: Redesign stpecpy() and stpeprintf()

Make them report truncation via errno and NULL.

Instead of having three possible returns (a pointer to the NUL byte, the
end of the array, or NULL), reduce it to two possible ones: one for
success, and one for error.

Use errno, which is a common way to signal the specific error, and thus
treat truncation as any other error.  This simplifies error handling
after these calls.  Also, if one misuses a pointer after truncation, the
results are better if the pointer is NULL: the program will easily
abort.  If we returned 'end', the program could more easily produce a
buffer overrun.

Suggested-by: Douglas McIlroy <douglas.mcilroy@dartmouth.edu>
Signed-off-by: Alejandro Colomar <alx@kernel.org>