eCryptfs: Support multiple keys per inode

This series of patches to eCryptfs introduces support for associating
multiple keys with individual files. When mounting, eCryptfs will
register one key for each key signature/identifier specified by an
ecryptfs_sig= mount option. These keys will all be used to encrypt the
FEK for each eCryptfs inode. This patch set also adds support for
searching the user's keyring for keys when opening existing files;
this allows the user to add keys to his keyring at any time, and
eCryptfs will use those keys to open files protected by those keys.