"ours" to avoid version bump

"Ours" to avoid version bump.

"ours" to avoid version bump.

"ours" merge to avoid version bump.

Closes ticket 28851.

In theory it would be better to detect this bug in advance, but this
approach is much simpler, and therefore safer to backport.

This closes tor issue 28973.

Linked connections aren't woken up by libevent due to I/O but rather
artificially so we can, by chunks, empty the spooled object(s).

Commit 5719dfb4 (in 0.3.4.1-alpha) made it
that the schedule_active_linked_connections_event would be only called once at
startup but this is wrong because then we would never go through again the
active linked connections.

Fortunately, everytime a new linked connection is created, the event is
activated and thus we would go through the active list again. On a busy relay,
this issue is mitigated by that but on a slower relays or bridge, a connection
could get stuck for a while until a new directory information request would
show up.

Fixes #28717, #28912

We now accumulate warning flags in a separate variable,
"TOR_WARNING_FLAGS", and write it to a "warning_flags" file.  Then
we test whether the compiler will accept "@warning_flags": if so, we
put "@warning_flags" in the CFLAGS; if not, we copy the contents of
"$TOR_WARNING_FLAGS" into the CFLAGS.

Closes ticket 28924.

But continue to run stem's unit and online tests during
"make test-stem-full".

Fixes bug 28568; bugfix on 0.2.6.3-alpha.