ppp.8: Fix typo cuad0 -> cuau0

Fix a typo that was missed probably because it was capitalized.
Mark up with .Pa and fix capitalization.

Specify that cuaUX and cuauX are for USB and UART attachments
respectively.

• Silence most lint warnings
• Update outdated links
• Minor style fixes
• Some igor(1) warnings were ignored on purpose

PR: 248359
Reviewed by: imp, mhorne, Alexander Ziaee <concussious.bugzilla@runbox.com>
Approved by: emaste (mentor)
Fixes: 839f11a4fe18 ("A number of places in the source tree still reference cuad.* after sio(4) was deprecated by uart(4).")
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1515

(cherry picked from commit 21ca690b5668d6b19b93327da3d4a423ea183fe4)