There are 58 non-compliant files. Mark them with the `notxml` keyword.
Also include the compliant ones in the GHA/checksrc xmllint CI job.
Also:
- delete XML prolog from the 3 test data files that had them.
- FILEFORMAT.md: document the `notxml` keyword.
- FILEFORMAT.md: fix a typo.
Follow-up to de49cc89ab#19470
Follow-up to f3095f0dbd#19528
Follow-up to 87ba80a6dfCloses#19595
Also:
- replace `manpage` with `man page`, add to `badwords.txt`.
- badwords.pl: import `-w` feature from curl-www, syncing the two
scripts fully.
- badwords.txt: import missing items from curl-www, syncing the two
files fully.
- pyspelling.words: drop `cURL` allowed word.
Closes#19468