Rename proxy_session.backend_feats to proxy_session.backend_features; we can
use more legible names here.
Adding necessary .gitignore.
Slightly better, i.e. more specific, way of identifying vi swap files.
Adding frontend TLS test for data transfers; currently the test does not
work due to the innards of the NetIO API.  Need to do some reworking of
the NetIO API to make this function properly.