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/API/Syscall.h

@@ -195,7 +195,8 @@ namespace Kernel {
     S(set_process_name)       \
     S(disown)                 \
     S(adjtime)                \
-    S(allocate_tls)
+    S(allocate_tls)           \
+    S(prctl)
 
 namespace Syscall {