setkey(8): Clarify language around AEAD ciphers.

AEAD ciphers for IPsec combine both encryption and authentication. As
such, ESP configurations using an AEAD cipher should not use a
seperate authentication algorithm via -A. However, this was not
apparent from the setkey manpage and 12.x and earlier did not perform
sufficient argument validation permitting users to pair an explicit -A
such as SHA256-HMAC with AES-GCM. (The result was a non-standard
combination of AES-CTR with the specified MAC, but with the wrong
initial block counter (and thus different keystream) compared to using
AES-CTR as the cipher.)

Attempt to clarify this in the manpage by explicitly calling out AEAD
ciphers (currently only AES-GCM) and noting that AEAD ciphers should
not use -A.

While here, explicitly note which authentication algorithms can be
used with esp vs esp-old. Also add subsection headings for the
different algorithm lists and tidy some language.

I did not convert the tables to column lists (Bl -column) though that
would probably be more correct than using literal blocks (Bd
-literal).

PR: 263379
Reviewed by: Pau Amma <pauamma@gundo.com>, markj
Differential Revision: https://reviews.freebsd.org/D34947

(cherry picked from commit e6dede145616ed8f98c629c23a2ba206b812c921)