Refer to AES-CBC as "aes-cbc" rather than "rijndael-cbc" for IPsec.

At this point, AES is the more common name for Rijndael128. setkey(8)
will still accept the old name, and old constants remain for
compatiblity.

Reviewed by: cem, bcr (manpages)
MFC after: 2 weeks
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D24964