start-stop-daemon: fix setting scheduler with musl

Linux does not provide a way to set scheduling parameters for a process,
only for threads, so musl sched_setscheduler() is only a stub and the
glibc implementation does "the wrong thing".

Use pthread_setschedparam instead for Linux.

ref: https://www.openwall.com/lists/musl/2016/03/01/5
fixes: #689

src/start-stop-daemon/start-stop-daemon.c

@@ -31,6 +31,7 @@
 #include <getopt.h>
 #include <limits.h>
 #include <grp.h>
+#include <pthread.h>
 #include <pwd.h>
 #include <sched.h>
 #include <signal.h>
@@ -1123,7 +1124,11 @@ int main(int argc, char **argv)
 			if (sched_prio == -1)
 				sched.sched_priority = sched_get_priority_min(scheduler_index);
 
+#ifdef __linux__
+			if (pthread_setschedparam(pthread_self(), scheduler_index, &sched))
+#else
 			if (sched_setscheduler(mypid, scheduler_index, &sched))
+#endif
 				eerrorx("Failed to set scheduler: %s", strerror(errno));
 		} else if (sched_prio != -1) {
 			const struct sched_param sched =  {.sched_priority = sched_prio};