pppd: Avoid use of strnlen (and strlen) in vslprintf

pppd: Avoid use of strnlen (and strlen) in vslprintf

Commit b311e98b ("pppd: Limit memory accessed by string formats with
max length specified") added calls to strnlen() in vslprintf().
Unfortunately, strnlen() is not provided in some standard C libraries.
This changes the code to avoid using strnlen().  Using the observation
that the number of characters we can use from the input string is
bounded by buflen, the number of bytes of output buffer available,
we can also avoid doing strlen() on a potentially long string.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>