Fix compiler warnings

src/pcscdaemon.c:578:10: warning: implicit conversion loses integer precision: 'ssize_t' (aka 'long') to 'int' [-Wshorten-64-to-32]
                        ret = read(pipefd[0], &buf, 1);
                            ~ ^~~~~~~~~~~~~~~~~~~~~~~~
src/pcscdaemon.c:851:6: warning: implicit conversion loses integer precision: 'ssize_t' (aka 'long') to 'int' [-Wshorten-64-to-32]
        r = write(signal_handler_fd[1], &sig, sizeof sig);
          ~ ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

src/pcscdaemon.c

@@ -177,7 +177,7 @@ static void *signal_thread(void *arg)
 
  while (true)
  {
- int r;
+ ssize_t r;
  int sig;
 
  r = read(signal_handler_fd[0], &sig, sizeof sig);
@@ -569,7 +569,7 @@ int main(int argc, char **argv)
  /* in the father */
  {
  char buf;
- int ret;
+ ssize_t ret;
 
  /* close write side */
  close(pipefd[1]);
@@ -846,7 +846,7 @@ static void clean_temp_files(void)
 
 static void signal_trap(int sig)
 {
- int r;
+ ssize_t r;
 
  r = write(signal_handler_fd[1], &sig, sizeof sig);
  if (r < 0)