Kernel+LibC: Add sys$disown() for disowning child processes

This syscall allows a parent process to disown a child process, setting
its parent PID to 0.

Unparented processes are automatically reaped by the kernel upon exit,
and no sys$waitid() is required. This will make it much nicer to do
spawn-and-forget which is common in the GUI environment.

Libraries/LibC/serenity.cpp

@@ -30,6 +30,12 @@
 
 extern "C" {
 
+int disown(pid_t pid)
+{
+    int rc = syscall(SC_disown, pid);
+    __RETURN_WITH_ERRNO(rc, rc, -1);
+}
+
 int module_load(const char* path, size_t path_length)
 {
     int rc = syscall(SC_module_load, path, path_length);