libnss_mxshadow: Return errno in errnop, too

From the glibc manual:

"Before the function returns with a failure code, the implementation
should store the value of the local errno variable in the variable
pointed to be errnop. This is important to guarantee the module working
in statically linked programs. The stored value must not be zero."

So do it.

libnss_mxshadow.c

@@ -28,17 +28,20 @@ enum nss_status _nss_mxshadow_getspnam_r(const char *name, struct spwd *spwd, ch
 
     int status = get_shadow_line_from_server_v2((char *)name, &line);
     if (status == -1) {
+        *errnop = errno;
         return NSS_STATUS_UNAVAIL;
     }
     size_t len = strlen(line);
     if (buflen < len+1) {
         errno = ERANGE;
+        *errnop = errno;
         return -1;
     }
     strcpy(buffer, line);
 
     if (buffer[0] == '\0') {
         errno = ENOENT;
+        *errnop = errno;
         return NSS_STATUS_NOTFOUND;
     }