! servlet: introduce `root-path = AUTO` and make it the default, fixes #701

Breaking change because it changes the default. This change is compatible
as long as root-path was set explicitly before. It will break user code
if the user code relies on root-path being set to "" which wasn't recommended
in the first place.
Merge pull request #801 from jrudolph/i/701-root-path-auto

! servlet: introduce `root-path = auto` and make it the default, fixes #701