Kernel+LibPthread: pthread_create handles pthread_attr_t

Add an initial implementation of pthread attributes for:
  * detach state (joinable, detached)
  * schedule params (just priority)
  * guard page size (as skeleton) (requires kernel support maybe?)
  * stack size and user-provided stack location (4 or 8 MB only, must be aligned)

Add some tests too, to the thread test program.

Also, LibC: Move pthread declarations to sys/types.h, where they belong.

Kernel/Syscall.h

@@ -248,6 +248,20 @@ struct SC_setsockopt_params {
     socklen_t value_size;
 };
 
+struct SC_create_thread_params {
+    unsigned int m_detach_state = 0; // JOINABLE or DETACHED
+    int m_schedule_priority = 2;     // ThreadPriority::Normal
+    // FIXME: Implment guard pages in create_thread (unreadable pages at "overflow" end of stack)
+    // "If an implementation rounds up the value of guardsize to a multiple of {PAGESIZE},
+    // a call to pthread_attr_getguardsize() specifying attr shall store in the guardsize
+    // parameter the guard size specified by the previous pthread_attr_setguardsize() function call"
+    // ... ok, if you say so posix. Guess we get to lie to people about guard page size
+    unsigned int m_guard_page_size = 0;          // Rounded up to PAGE_SIZE
+    unsigned int m_reported_guard_page_size = 0; // The lie we tell callers
+    unsigned int m_stack_size = 4 * MB;          // Default PTHREAD_STACK_MIN
+    void* m_stack_location = nullptr;            // nullptr means any, o.w. process virtual address
+};
+
 void initialize();
 int sync();