transaction: deprecate ambiguous install function

install had overloaded meaning that wasn't properly documented and crappy default value to boot.

instead untangle the use cases on an API level at least to improve expressiveness:

• installLinkId may be used to install a specific linkid and consumes an unsigned of that id. it has no default id and ought to only be used when a specific id is known (e.g. the user picked one in a UI or something)
• installLatest installs the latest downloadable entity in whatever way knewstuff finds useful. this is the primary operation mode for when the id is not known and the application simply wants to install whichever one looks good

the backing code has been slightly refactored to reduce nesting and also complexity (a bit anyway)