HAVE READ ACCESS TO THE SOURCE KEYRING

DESCRIPTION
===========
If you attempt to perform a keyring migration with an
operating system user that does not have access to write to
the source keyring file, then the migration fails.

ANALYSIS
========
Currently the keyring file is openeded in RW|C mode, and is
agnostic of its type (source or dest). So when it doesn't
have Write privilege on the file, the migration fails. In
such a case migration should be allowed even if the user
has got Read privilege on the file.

FIX
===
Introducing a new internal variable that corresponds to if
the file has to be opened in Read_only or RW mode. This
variable is set in Migrate_keyring::init() only for source
plugin (thus for dest it's value defaults to 0) and passed
along with other migration params.

On reading this value as set,
check_if_keyring_file_can_be_opened_or_created() and
get_serialized_object() open the file in Read only mode,
else in RW|C mode