Kernel+LibC: Switch isatty() to use a fcntl()

We would want it to work with only stdio pledged.

Kernel/Process.cpp

@@ -1845,6 +1845,8 @@ int Process::sys$fcntl(int fd, int cmd, u32 arg)
     case F_SETFL:
         description->set_file_flags(arg);
         break;
+    case F_ISTTY:
+        return description->is_tty();
     default:
         return -EINVAL;
     }

Kernel/UnixTypes.h

@@ -74,6 +74,7 @@
 #define F_SETFD 2
 #define F_GETFL 3
 #define F_SETFL 4
+#define F_ISTTY 5
 
 #define FD_CLOEXEC 1
 

Libraries/LibC/fcntl.h

@@ -36,6 +36,7 @@ __BEGIN_DECLS
 #define F_SETFD 2
 #define F_GETFL 3
 #define F_SETFL 4
+#define F_ISTTY 5
 
 #define FD_CLOEXEC 1
 

Libraries/LibC/unistd.cpp

@@ -413,8 +413,7 @@ int rmdir(const char* pathname)
 
 int isatty(int fd)
 {
-    struct termios dummy;
-    return tcgetattr(fd, &dummy) == 0;
+    return fcntl(fd, F_ISTTY);
 }
 
 int dup(int old_fd)