Fix for 12837: Duration's get and individual unit getters are bug-prone.

Experience has shown that most people (who don't read the documentation)
seem to expect that get and the indivdual getters return what total
returns, causing bugs. So, this commit renames get to getOnly (leaving
get as a deprecated alias to getOnly) and deprecates the individual unit
getters, forcing folks to use total or getOnly explicitly. It's likely
that most code that will have to change because of these changes is
broken anyway, so this will catch bugs.