Fix compiler warnings (-Wunused-parameter) in various functions

Fixes the -Wunused-parameter warning in:
* group/group_signal.c: group_signal()
* irq/irq_unexpectedisr.c: irq_unexpected_isr()
* task/task_spawn.c: nxtask_spawn_proxy()
* timer/timer_getoverrun.c: timer_getoverrun()
* misc/dev_null.c: devnull_read(), devnull_write(), devnull_poll()
* misc/dev_zero.c: devzero_read(), devzero_write(), devzero_poll()
* syslog/syslog_channel.c: syslog_default_write()
* syslog/syslog_device.c: syslog_dev_flush()
* grp/lib_initgroups.c: initgroups()
* misc/lib_mknod.c: mknod()
* misc/lib_glob.c: ignore_err()
* pthread/pthread_barrierinit.c: pthread_barrier_init()
* pthread/pthread_atfork.c: pthread_atfork()
* semaphore/sem_init.c: nxsem_init()
* stream/lib_nullinstream.c: nullinstream_getc()
* stream/lib_nulloutstream.c: nulloutstream_putc()
* stream/lib_libnoflush.c: lib_noflush()
* stream/lib_libsnoflush.c: lib_snoflush()
* string/lib_strerror.c: strerror()
* time/lib_gettimeofday.c: gettimeofday()
* time/lib_settimeofday.c: settimeofday()
* unistd/lib_pathconf.c: fpathconf(), pathconf()
* unistd/lib_getrusage.c: getrusage()
* unistd/lib_setrlimit.c: setrlimit()
* unistd/lib_getrlimit.c: getrlimit()
* unistd/lib_setpriority.c: setpriority()

libs/libc/string/lib_strerror.c

@@ -365,6 +365,8 @@ FAR char *strerror(int errnum)
         }
     }
   while (ndxlow <= ndxhi);
+#else
+  UNUSED(errnum);
 #endif
   return "Unknown error";
 }