Bug#38477: my_pthread_setprio can change dispatch class on Solaris, not just priority

The problem is that the function used by the server to increase
the thread's priority (pthread_setschedparam) has the unintended
side-effect of changing the calling thread scheduling policy,
possibly overwriting a scheduling policy set by a sysadmin.

The solution is to rely on the pthread_setschedprio function, if
available, as it only changes the scheduling priority and does not
change the scheduling policy. This function is usually available on
Solaris and Linux, but it use won't work by default on Linux as the
the default scheduling policy only accepts a static priority 0 -- this
is acceptable for now as priority changing on Linux is broken anyway.

configure.in:
  Check for the existence of the pthread_setschedprio function.
include/my_pthread.h:
  Use the pthread_setschedprio function to set the thread priority
  if the function is available.

configure.in

@@ -2040,7 +2040,7 @@ AC_CHECK_FUNCS(alarm bcmp bfill bmove bsearch bzero \
   mkstemp mlockall perror poll pread pthread_attr_create mmap mmap64 getpagesize \
   pthread_attr_getstacksize pthread_attr_setprio pthread_attr_setschedparam \
   pthread_attr_setstacksize pthread_condattr_create pthread_getsequence_np \
-  pthread_key_delete pthread_rwlock_rdlock pthread_setprio \
+  pthread_key_delete pthread_rwlock_rdlock pthread_setprio pthread_setschedprio \
   pthread_setprio_np pthread_setschedparam pthread_sigmask readlink \
   realpath rename rint rwlock_init setupterm \
   shmget shmat shmdt shmctl sigaction sigemptyset sigaddset \

include/my_pthread.h

@@ -279,6 +279,8 @@ int sigwait(sigset_t *setp, int *sigp);		/* Use our implemention */
 #define my_pthread_setprio(A,B) pthread_setprio_np((A),(B))
 #elif defined(HAVE_PTHREAD_SETPRIO)
 #define my_pthread_setprio(A,B) pthread_setprio((A),(B))
+#elif defined(HAVE_PTHREAD_SETSCHEDPRIO)
+#define my_pthread_setprio(A,B) pthread_setschedprio((A),(B))
 #else
 extern void my_pthread_setprio(pthread_t thread_id,int prior);
 #endif