Fix vfprintf and derived functions not handling write errors (#798)

The C standard states:
> The fprintf function returns the number of characters transmitted,
> or a negative value if an output or encoding error occurred or if
> the implementation does not support a specified width length
> modifier.
- C Standard, 7.23.6.1.15. The fprintf function

However, cosmopolitan fails to return a negative value in the case of
an output error, meaning that a program such as:

 #include <stdio.h>

int main()
{
    FILE *fp = fopen("/dev/full", "w");

    setbuf(fp, NULL);
    printf("fprintf: %d\n", fprintf(fp, "test\n"));
    printf("fflush: %d\n", fflush(fp));
}

will, under cosmopolitan, print that no error occured in either of the
calls to fprintf and fflush.

This patch fixes this, along with the associated GitHub issue,
https://github.com/jart/cosmopolitan/issues/784

libc/stdio/vfprintf_unlocked.c

@@ -44,7 +44,7 @@ static int vfprintfputchar(const char *s, struct state *t, size_t n) {
   } else {
     rc = 0;
   }
-  return 0;
+  return rc;
 }
 
 /**