Kernel+LibC: Introduce a "dumpable" flag for processes

This new flag controls two things:
- Whether the kernel will generate core dumps for the process
- Whether the EUID:EGID should own the process's files in /proc

Processes are automatically made non-dumpable when their EUID or EGID is
changed, either via syscalls that specifically modify those ID's, or via
sys$execve(), when a set-uid or set-gid program is executed.

A process can change its own dumpable flag at any time by calling the
new sys$prctl(PR_SET_DUMPABLE) syscall.

Fixes #4504.

Kernel/CoreDump.cpp

@@ -42,6 +42,11 @@ namespace Kernel {
 
 OwnPtr<CoreDump> CoreDump::create(Process& process, const String& output_path)
 {
+    if (!process.is_dumpable()) {
+        dbgln("Refusing to generate CoreDump for non-dumpable process {}", process.pid().value());
+        return nullptr;
+    }
+
     auto fd = create_target_file(process, output_path);
     if (!fd)
         return nullptr;