Kernel: Simplify Process factory functions

- Instead of taking the first new thread as an out-parameter, we now
  bundle the process and its first thread in a struct and use that
  as the return value.

- Make all Process factory functions return ErrorOr. Use this to convert
  some places to more TRY().

- Drop the "try_" prefix on Process factory functions.

Kernel/FileSystem/Plan9FS/FileSystem.cpp

@@ -353,10 +353,11 @@ void Plan9FS::ensure_thread()
         auto process_name = KString::try_create("Plan9FS"sv);
         if (process_name.is_error())
             TODO();
-        (void)Process::create_kernel_process(m_thread, process_name.release_value(), [&]() {
+        auto [_, thread] = Process::create_kernel_process(process_name.release_value(), [&]() {
             thread_main();
             m_thread_running.store(false, AK::MemoryOrder::memory_order_release);
-        });
+        }).release_value_but_fixme_should_propagate_errors();
+        m_thread = move(thread);
     }
 }