Cygwin: sched_setscheduler: allow changes of the priority

Behave like sched_setparam() if the requested policy is identical
to the fixed value (SCHED_FIFO) returned by sched_getscheduler().

Fixes: 9a08b2c02e ("* sched.cc: New file.  Implement sched*.")
Signed-off-by: Christian Franke <christian.franke@t-online.de>

winsup/cygwin/release/3.5.5

@@ -45,3 +45,6 @@ Fixes:
 
 - Fix segfault in sigtimedwait() when using timeout.
   Addresses: https://cygwin.com/pipermail/cygwin/2024-November/256762.html
+
+- sched_setscheduler(2) allows to change the priority if the policy is
+  equal to the value returned by sched_getscheduler(2).

winsup/cygwin/sched.cc

@@ -402,8 +402,11 @@ int
 sched_setscheduler (pid_t pid, int policy,
 		    const struct sched_param *param)
 {
+  if (policy == SCHED_FIFO) /* returned by sched_getscheduler. */
+    return sched_setparam (pid, param);
+
   /* on win32, you can't change the scheduler. Doh! */
-  set_errno (ENOSYS);
+  set_errno (EINVAL);
   return -1;
 }