Create a 'user links' menu + page template variable

Split from #273137: Split Navigation to User and Administration menu - secondary links isn't the ideal container for 'My Account', 'Logout' etc. - since it's also got the special handling as child links of primary links etc. - so we need a new user links menu and associated template variable which can be used to put these consistently in the top right of themes (by default at least). In Garland, this conflicts with the primary/secondary links so it'll require theme changes - the menu changes are pretty simple and already in one of the patches on the previous issue.