sched: Map SCHED_OTHER to SCHED_FIFO or SCHED_RR

this behaviour is explicitly specified here:
https://pubs.opengroup.org/onlinepubs/009695399/functions/xsh_chap02_08.html
and map SCHED_NORMAL to SCHED_OTHER like Linux:
https://man7.org/linux/man-pages/man7/sched.7.html

Signed-off-by: Xiang Xiao <xiaoxiang@xiaomi.com>

include/nuttx/pthread.h

@@ -37,11 +37,7 @@
 
 /* Default pthread attribute initializer */
 
-#if CONFIG_RR_INTERVAL == 0
-#  define PTHREAD_DEFAULT_POLICY SCHED_FIFO
-#else
-#  define PTHREAD_DEFAULT_POLICY SCHED_RR
-#endif
+#define PTHREAD_DEFAULT_POLICY SCHED_NORMAL
 
 /* A lot of hassle to use the old-fashioned struct initializers.  But this
  * gives us backward compatibility with some very old compilers.

include/sched.h

@@ -41,10 +41,11 @@
 
 /* POSIX-like scheduling policies */
 
+#define SCHED_NORMAL              0  /* Alias to SCHED_OTHER */
+#define SCHED_OTHER               0  /* Map to SCHED_FIFO or SCHED_RR */
 #define SCHED_FIFO                1  /* FIFO priority scheduling policy */
 #define SCHED_RR                  2  /* Round robin scheduling policy */
 #define SCHED_SPORADIC            3  /* Sporadic scheduling policy */
-#define SCHED_OTHER               4  /* Not supported */
 
 /* Maximum number of SCHED_SPORADIC replenishments */