Kernel: Implement the killpg() syscall

Author: https://github.com/bugaevc
Commit: d29a078ab3
Pull-request: https://github.com/SerenityOS/serenity/pull/779

Kernel/Process.h

@@ -315,6 +315,9 @@ private:
 
     KResultOr<Vector<String>> find_shebang_interpreter_for_executable(const String& executable_path);
 
+    KResult do_kill(Process&, int signal);
+    KResult do_killpg(pid_t pgrp, int signal);
+
     Thread* m_main_thread { nullptr };
 
     RefPtr<PageDirectory> m_page_directory;