Skip tcgetattr() in patty_kiss_tnc_new_fd() w/ptmx

Modify patty_kiss_tnc_new_fd() to avoid tcgetattr()/tcsetattr() when
passed a file descriptor for a PTY master, to address incompatibility
with BSD kernelspace Unix98 PTY implementations

src/kiss.c

@@ -52,7 +52,7 @@ patty_kiss_tnc *patty_kiss_tnc_new_fd(int fd) {
         goto error_malloc_buf;
     }
 
-    if (isatty(fd)) {
+    if (isatty(fd) && ptsname(fd) == NULL) {
         if (tcgetattr(fd, &tnc->attrs) < 0) {
             goto error_tcgetattr;
         }