pipeline: fix mistakenly trying to pipeline POSTs

The function IsPipeliningPossible() would return TRUE if either
pipelining OR HTTP/2 were possible on a connection, which would lead to
it returning TRUE even for POSTs on HTTP/1 connections.

It now returns a bitmask so that the caller can differentiate which kind
the connection allows.

Fixes #1481
Closes #1483
Reported-by: stootill at github