* g10.c (main): Add --symmetric --encrypt command. This generates a

message that can be decrypted via a passphrase or public key system.

* main.h, encode.c (encode_seskey): Allow passing in an already-created
session key dek. (encode_simple): Use the actual symmetric cipher when
encrypting a session key for a symmetric message. (encode_crypt): Add a
flag to trigger a hybrid mode that can be decrypted via a passphrase or a
pk.  Change all callers.

* mainproc.c (symkey_decrypt_sesskey): There is no way to tell the
difference here between a bad passphrase and a cipher algorithm that we
don't have, so use a error message that makes that clear. Use the actual
list of ciphers when checking whether a cipher is invalid.  Return error
if the decrypted cipher algorithm is invalid. (proc_symkey_enc): In a
mixed passphrase/pk message, if a valid dek already exists from decrypting
via pk, do not try to process the passphrase. (proc_symkey_enc): Indicate
when we're decrypting a session key as opposed to decrypting data.  If a
passphrase is invalid, discard the dek so we'll keep trying.

g10/main.h

@@ -112,7 +112,7 @@ void display_online_help( const char *keyword );
 /*-- encode.c --*/
 int encode_symmetric( const char *filename );
 int encode_store( const char *filename );
-int encode_crypt( const char *filename, STRLIST remusr );
+int encode_crypt( const char *filename, STRLIST remusr, int use_symkey );
 void encode_crypt_files(int nfiles, char **files, STRLIST remusr);
 int encrypt_filter( void *opaque, int control,
 		    IOBUF a, byte *buf, size_t *ret_len);