Make a real SoupSSLCredentials type rather than just using gpointer

Make a real SoupSSLCredentials type rather than just using gpointer

	* libsoup/soup-ssl.h: Make a real SoupSSLCredentials type rather
	than just using gpointer

	* libsoup/soup-server.c (SoupServerPrivate): use it

	* libsoup/soup-session.c (SoupSessionPrivate): use it

	* libsoup/soup-gnutls.c: Use it, and consistently use "creds"
	rather than "cred" as the abbreviation for "credentials".

	* docs/reference/libsoup-sections.txt:
	* docs/reference/tmpl/soup-misc.sgml:
	* docs/reference/tmpl/soup-ssl.sgml: update

svn path=/trunk/; revision=924