Avoid an unnecessary thread creation at rustdoc startup.

rustdoc's `main()` immediately spawns a thread, M, with a large stack
(16MiB or 32MiB) on which it runs `main_args()`. `main_args()` does a
small amount of options processing and then calls
`setup_callbacks_and_run_in_default_thread_pool_with_globals()`, which
spawns it own thread, and M is not used further.

So, thread M seems unnecessary. However, it does serve a purpose: if the
options processing in `main_args()` panics, that panic is caught when M
is joined. So M can't simply be removed.

However, `main_options()`, which is called by `main_args()`, has a
`catch_fatal_errors()` call within it. We can move that call to `main()`
and change it to the very similar `catch_with_exit_code()`. With that in
place, M can be removed, and panics from options processing will still
be caught appropriately.

Even better, this makes rustdoc's `main()` match rustc's `main()`, which
also uses `catch_with_exit_code()`.

(Also note that the use of a 16MiB/32MiB stack was eliminated from rustc
in #55617.)

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