'openssl passwd' command can now compute AIX MD5-based passwords hashes.

The difference between the AIX MD5 password algorithm and the standard MD5
password algorithm is that in AIX there is no magic string while in the
standard MD5 password algorithm the magic string is "$1$"

Documentation of '-aixmd5' option of 'openssl passwd' command is added.

1 test is added in test/recipes/20-test-passwd.t

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2251)