Fix newlines in subject being shown with ?

[Yutsuten]

• What does this PR do?

1. Keeps env->subject with the original content, the previous code was replacing LF characters with ?.
   1. Other fields may be affected, but I think that if any additional fixes become needed, it should be done in format_string() or similar
2. Sanitize env->subject into env->disp_subj by replacing whitespace characters (iswspace()) with a space character.

I also tested with and without a subjectrx configuration, seems to be working as expected.

There are also some side changes as discussed in #4276

1. #ifdef not needed anymore in expando/format.c

As iswspace() is a bigger set than iswblank(), we can drop iswblank().

2. Return value of subjrx_apply_mods from bool to void. Return value is not used anywhere

• Screenshots (if relevant)

Subject with newlines before the change:

???      ? ? ?    ?  ?  ?  ?   ??Google Cloud Platform & APIs...

Subject with newlines after the change:

                                 Google Cloud Platform & APIs...

• What are the relevant issue numbers?
  Subject with non-printable characters or new line is shown with ? #4276

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This is my first PR with C language related changes. Please tell me if there are any security or performance issues in my code.

Thank you @flatcap for giving me so much information in the issue.

Also please tell me if it is better to squash every commit into one at the end of the reviews.

[flatcap]

Thanks!
I shall have a read and a play as soon as I can.