sm: Support decryption of password based encryption (pwri)

* sm/decrypt.c (string_from_gcry_buffer): New.
(pwri_parse_pbkdf2): New.
(pwri_decrypt): New.
(prepare_decryption): Support pwri.
(gpgsm_decrypt): Test for PWRI.  Move IS_DE_VS flag to DFPARM.
* common/sexputil.c (cipher_mode_to_string): New.

Note that this is not finished because we need to implement a password
callback. For now "abc" is used as passwort.

Latest libksba is also required to return the required info.

Backported-from-master: 02029f9eab87e9fd667829dfb083846275576398

Note that this change also prints the used algorithm and shows some
existsing diagnostics only in verbose mode.

• Signed-off-by: Werner Koch <wk@gnupg.org>