Fix handling of large strings in strncat.

There were two issues:
*If bit 15 of the n value was set, the second string would not be copied.
*If the length of the second string was 64K or more, it would not be copied properly because the pointers were not updated.

This test program demonstrates both issues:

#pragma memorymodel 1
#include <stdlib.h>
#include <string.h>
#include <stdio.h>
#define LEN2 100000
int main(void) {
        char *s1 = malloc(LEN2+2);
        char *s2 = malloc(LEN2+1);
        if (!s1 || !s2)
                return 0;
        for (unsigned long i = 0; i < LEN2; i++)
                s2[i] = '0' + (i & 0x07);
        strcpy(s1,"a");
        strncat(s1, s2, LEN2);
        puts(s1);
        printf("len = %zu\n", strlen(s1));
}

string.asm

@@ -1021,12 +1021,14 @@ lb2a     short M                        copy characters 'til the null is found
          ldy   #0
          ldx   n
          beq   lb4
-         bmi   lb4
 lb3      lda   [s2],Y
          sta   [s1],Y
          beq   lb4
          iny
-         dex
+         bne   lb3a
+         inc   s1+2
+         inc   s2+2
+lb3a     dex
          bne   lb3
          lda   n+2
          beq   lb4