password utils: improve --help, make DEFAULT_PASSWD_ALGO visible if CHPASSWD

password utils: improve --help, make DEFAULT_PASSWD_ALGO visible if CHPASSWD

Was:
    $ cryptpw --help
    ...
    Print crypt(3) hashed PASSWORD

        -P,--password-fd=N	Read password from fd N
        -m,--method=TYPE	Encryption method
        -S,--salt=SALT

User: "What methods exist? which one os default?"

Now:
    Print crypt(3) hashed PASSWORD

        -P,--password-fd N	Read password from fd N
        -m,--method TYPE	des,md5,sha256/512 (default des)
        -S,--salt SALT

Signed-off-by: Denys Vlasenko <vda.linux@googlemail.com>