daemon: Free presentation identity when finished with it

The update_account_object function reads an account's presentation
identity from the accounts configuration.  GKeyFile returns a
duped string and so the presentation identity needs to be freed.

https://bugzilla.gnome.org/show_bug.cgi?id=739590
kerberos: Plug leak in error handling

It's a little tedious to convert a kerberos error into a GError,
so there's a helper function to do it: set_error_from_krb5_error_code

That helper function leaks the format string use for constructing the
GError's message.

This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=739590
identity: plug leak in identity service

When the identity service first starts up, it gets a list
of preexisting kerberos principals and then exports them
over the bus.

The object path used for the exported objects is getting leaked.
This commit fixes that.

https://bugzilla.gnome.org/show_bug.cgi?id=739590
kerberos: drop weird, superfluous assignment in query function

goa_kerberos_identity_inquiry_answer_query has this strange code:

g_return_if_fail (inquiry == kerberos_query->inquiry);
...
inquiry = kerberos_query->inquiry;

It's completely redundant and makes no sense so drop it.

https://bugzilla.gnome.org/show_bug.cgi?id=739593
kerberos: Zero initialize operation structure

We need zero initialize the structure so we don't end up
freeing unused members when cleaning up the operation.

https://bugzilla.gnome.org/show_bug.cgi?id=739593