Kernel: Move E2BIG calculation from Thread to Process

Thread::make_userspace_stack_for_main_thread is only ever called from
Process::do_exec, after all the fun ELF loading and TSS setup has
occured.

The calculations in there that check if the combined argv + envp
size will exceed the default stack size are not used in the rest of
the stack setup. So, it should be safe to move this to the beginning
of do_exec and bail early with -E2BIG, just like the man pages say.

Additionally, advertise this limit in limits.h to be a good POSIX.1
citizen. :)

Kernel/Process.cpp

@@ -33,6 +33,7 @@
 #include <Kernel/StdLib.h>
 #include <Kernel/Syscall.h>
 #include <Kernel/TTY/MasterPTY.h>
+#include <Kernel/Thread.h>
 #include <Kernel/VM/InodeVMObject.h>
 #include <LibC/errno_numbers.h>
 #include <LibC/signal_numbers.h>
@@ -379,6 +380,18 @@ int Process::do_exec(String path, Vector<String> arguments, Vector<String> envir
         ASSERT_NOT_REACHED();
     }
 
+    size_t total_blob_size = 0;
+    for (auto& a : arguments)
+        total_blob_size += a.length() + 1;
+    for (auto& e : environment)
+        total_blob_size += e.length() + 1;
+
+    size_t total_meta_size = sizeof(char*) * (arguments.size() + 1) + sizeof(char*) * (environment.size() + 1);
+
+    // FIXME: How much stack space does process startup need?
+    if ((total_blob_size + total_meta_size) >= Thread::default_userspace_stack_size)
+        return -E2BIG;
+
     auto parts = path.split('/');
     if (parts.is_empty())
         return -ENOENT;