add support for thread scheduling (POSIX TPS option)

linux's sched_* syscalls actually implement the TPS (thread
scheduling) functionality, not the PS (process scheduling)
functionality which the sched_* functions are supposed to have.
omitting support for the PS option (and having the sched_* interfaces
fail with ENOSYS rather than omitting them, since some broken software
assumes they exist) seems to be the only conforming way to do this on
linux.

src/internal/pthread_impl.h

@@ -40,6 +40,8 @@ struct pthread {
 	locale_t locale;
 	int killlock[2];
 	int exitlock[2];
+	int startlock[2];
+	unsigned long sigmask[__SYSCALL_SSLEN/sizeof(long)];
 };
 
 struct __timer {
@@ -53,6 +55,9 @@ struct __timer {
 #define _a_guardsize __u.__s[1]
 #define _a_stackaddr __u.__s[2]
 #define _a_detach __u.__i[3*__SU+0]
+#define _a_sched __u.__i[3*__SU+1]
+#define _a_policy __u.__i[3*__SU+2]
+#define _a_prio __u.__i[3*__SU+3]
 #define _m_type __u.__i[0]
 #define _m_lock __u.__i[1]
 #define _m_waiters __u.__i[2]