LibGUI+Kernel: Add a GLock class (userspace mutex.)

It's basically a userspace port of the kernel's Lock class.
Added gettid() and donate() syscalls to support the timeslice donation
feature we already enjoyed in the kernel.

Kernel/Process.cpp

@@ -2461,3 +2461,27 @@ int Process::sys$create_thread(int(*entry)(void*), void* argument)
     thread->set_state(Thread::State::Runnable);
     return 0;
 }
+
+int Process::sys$gettid()
+{
+    return current->tid();
+}
+
+int Process::sys$donate(int tid)
+{
+    if (tid < 0)
+        return -EINVAL;
+    InterruptDisabler disabler;
+    Thread* beneficiary = nullptr;
+    for_each_thread([&] (Thread& thread) {
+        if (thread.tid() == tid) {
+            beneficiary = &thread;
+            return IterationDecision::Abort;
+        }
+        return IterationDecision::Continue;
+    });
+    if (!beneficiary)
+        return -ENOTHREAD;
+    Scheduler::donate_to(beneficiary, "sys$donate");
+    return 0;
+}

Kernel/Process.h

@@ -97,6 +97,8 @@ public:
     void die();
     void finalize();
 
+    int sys$gettid();
+    int sys$donate(int tid);
     pid_t sys$setsid();
     pid_t sys$getsid(pid_t);
     int sys$setpgid(pid_t pid, pid_t pgid);

Kernel/Syscall.cpp

@@ -57,6 +57,10 @@ static dword handle(RegisterDump& regs, dword function, dword arg1, dword arg2,
     case Syscall::SC_yield:
         Scheduler::yield();
         break;
+    case Syscall::SC_donate:
+        return current->process().sys$donate((int)arg1);
+    case Syscall::SC_gettid:
+        return current->process().sys$gettid();
     case Syscall::SC_putch:
         Console::the().put_char(arg1 & 0xff);
         break;

Kernel/Syscall.h

@@ -93,6 +93,8 @@
     __ENUMERATE_SYSCALL(getsockopt) \
     __ENUMERATE_SYSCALL(setsockopt) \
     __ENUMERATE_SYSCALL(create_thread) \
+    __ENUMERATE_SYSCALL(gettid) \
+    __ENUMERATE_SYSCALL(donate) \
 
 
 namespace Syscall {