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.

LibC/string.cpp

@@ -312,6 +312,7 @@ const char* sys_errlist[] = {
     "Timed out",
     "Wrong protocol type",
     "Operation in progress",
+    "No such thread",
     "The highest errno +1 :^)",
 };