* common/name-value.c (struct name_value_container): Add field
'private_key_mode'.
(my_error): New.  Use instead of gpg_error.
(nvc_new_private_key): New.
(nve_release): Add arg 'private_key_mode'.
(nvc_release): Call nve_release with private_key_mode flag.
(nvc_delete): Ditto.
(_nvc_add): Do no special case "Key:" in non-private_key_mode.
(nvc_get_private_key): Return error in non-private_key_mode.
(nvc_set_private_key): Ditto.
(nvc_parse):  Factor all code out to ...
(do_nvc_parse): new.  Add arg 'for_private_key'.
(nvc_parse_private_key): New.
* agent/findkey.c (write_extended_private_key): Replace nvc_parse by
nvc_parse_private_key.
(read_key_file): Ditto.
* common/t-name-value.c (private_key_mode): New variable.
(my_nvc_new): New.  Replace all callers.
(test_key_extraction): Take mode in account.
(run_tests): Ditto.
(run_modification_tests): Ditto.
(parse): Ditto.
(main): Add option --parse and rename --parse to --parse-key.