Add base template processing.

Base templates are templates that are used to inherit from.  They can
loosely be compared with parent class inheritance in object orientation.
They can be used for the same purpose as the variables with multi-field
strings are used in old-style string configurations.

Base templates are declared with the base_templates configuration
attribute, like so:

	"example_target" => {
		base_templates => [ "x86_asm", ... ]
		...
	}

Note: The value of base_templates MUST be an array reference (an array
enclosed in square brackets).

Any configuration target can be used as a base template by another.  It
is also possible to have a target that's a pure template and not meant to
be used directly as a configuration target.  Such a target is marked with
the template configuration attribute, like so:

	"example_template" => {
		template => 1,
		cc => "mycc",
		...
	},

As part of this commit, all variables with multi-field strings have been
translated to pure templates.  The variables currently remain since we
can't expect people to shift to hash table configurations immediately.

Reviewed-by: Andy Polyakov <appro@openssl.org>