strsep final separator handling

Hello,

I think I have identified a small difference between newlib strsep
and GNU/BSD strsep's handling of separators at the end of a string.

GNU and BSD strsep will return a pointer to the null terminating
character when asked to process a separator ending immediately before
a null-termination character. newlib strsep seems to return NULL instead.

Similarly, when presented with an empty string, GNU and BSD strsep will
return a pointer to the null terminating character, but newlib strsep
will return NULL.

I found this on _NEWLIB_VERSION "2.2.0" built for ARMv7-AR, but
I do not believe the implementation has changed since then when 
comparing to the master branch in git.

Here is a short example program that prints Found: three times 
when using GNU/BSD strsep, but only twice using newlib strsep:

#include <stdio.h>
#include <stdlib.h>
#include <string.h>

#define TEST_STRING ",,"
#define DELIM ","

int main()
{
char * str = strdup(TEST_STRING);
char * found;
char * strwalker = str;

while( (found = strsep(&strwalker,DELIM)) != NULL )
    {
    printf("Found:%s\n",found);
    }

free(str);
}

-Richard