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/CMakeLists.txt

@@ -136,6 +136,7 @@ set(KERNEL_SOURCES
     Syscalls/perf_event.cpp
     Syscalls/pipe.cpp
     Syscalls/pledge.cpp
+    Syscalls/prctl.cpp
     Syscalls/process.cpp
     Syscalls/profiling.cpp
     Syscalls/ptrace.cpp