ima: new templates management mechanism

The original 'ima' template is fixed length, containing the filedata hash
and pathname.  The filedata hash is limited to 20 bytes (md5/sha1).  The
pathname is a null terminated string, limited to 255 characters.  To
overcome these limitations and to add additional file metadata, it is
necessary to extend the current version of IMA by defining additional
templates.

The main reason to introduce this feature is that, each time a new
template is defined, the functions that generate and display the
measurement list would include the code for handling a new format and,
thus, would significantly grow over time.

This patch set solves this problem by separating the template management
from the remaining IMA code. The core of this solution is the definition
of two new data structures: a template descriptor, to determine which
information should be included in the measurement list, and a template
field, to generate and display data of a given type.

To define a new template ...