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strndup: don't assume buffer is terminated

Using strlen in strndup will walk past the first
n bytes up to the terminator, which may not be
present. This is not what we want.

While we're here, do some cleanups.

--- a/sys/src/ape/lib/ap/gen/strndup.c

+++ b/sys/src/ape/lib/ap/gen/strndup.c

@@ -8,13 +8,11 @@

 	int n;

 	char *np;

-	n = strlen(p)+1;

-	if(n > max)

-		n = max+1;

-	np = malloc(n);

+	n = strnlen(p, max);

+	np = malloc(n+1);

 	if(!np)

-		return nil;

-	memmove(np, p, n);

-	np[n-1] = 0;

+		return NULL;

+	memcpy(np, p, n);

+	np[n] = 0;

 	return np;

 }