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/Process.h

@@ -192,6 +192,9 @@ public:
     gid_t sgid() const { return m_sgid; }
     ProcessID ppid() const { return m_ppid; }
 
+    bool is_dumpable() const { return m_dumpable; }
+    void set_dumpable(bool dumpable) { m_dumpable = dumpable; }
+
     ThreadID exec_tid() const { return m_exec_tid; }
 
     mode_t umask() const { return m_umask; }
@@ -371,6 +374,7 @@ public:
     long sys$sysconf(int name);
     int sys$disown(ProcessID);
     void* sys$allocate_tls(size_t);
+    int sys$prctl(int option, FlatPtr arg1, FlatPtr arg2);
 
     template<bool sockname, typename Params>
     int get_sock_or_peer_name(const Params&);
@@ -627,6 +631,8 @@ private:
     ProcessID m_ppid { 0 };
     mode_t m_umask { 022 };
 
+    bool m_dumpable { true };
+
     Vector<gid_t> m_extra_gids;
 
     WeakPtr<Region> m_master_tls_region;