Fix broken node structure in user manual.

2013-07-09  Didier Verna  <didier@didierverna.net>

	* doc/user.texi: Add missing backlink to the Supported Platforms
	node.
Improve some DECLT keyword arguments and their initial values.

2013-07-09  Didier Verna  <didier@didierverna.net>

	* package.lisp (:com.dvlsoft.declt): Import :SYSTEM-LONG-NAME from
	ASDF.
	* src/declt.lisp: (declt): Rename some keyword arguments (subtitle
	-> tagline, author -> maintainer, email -> contact) and update
	their initialization.
	(render-header):
	* doc/user.texi (Usage): Update accordingly.
	* INSTALL: Regenerate.
Document ASDF 3's system long name slot.

2013-07-09  Didier Verna  <didier@didierverna.net>

	* src/doc/asdf.lisp (document (asdf:system)): Document the
	system's long name.