Kernel: Remove ProcessInspectionHandle and make Process RefCounted

By making the Process class RefCounted we don't really need
ProcessInspectionHandle anymore. This also fixes some race
conditions where a Process may be deleted while still being
used by ProcFS.

Also make sure to acquire the Process' lock when accessing
regions.

Last but not least, there's no reason why a thread can't be
scheduled while being inspected, though in practice it won't
happen anyway because the scheduler lock is held at the same
time.

Kernel/Syscalls/kill.cpp

@@ -139,7 +139,7 @@ int Process::sys$kill(pid_t pid, int signal)
         return do_killself(signal);
     }
     ScopedSpinLock lock(g_processes_lock);
-    auto* peer = Process::from_pid(pid);
+    auto peer = Process::from_pid(pid);
     if (!peer)
         return -ESRCH;
     return do_kill(*peer, signal);