Code from dgoulet.

This shouldn't be possible while Tor is running, but the tests can
hit this code.  Rather than force the tests to add a dummy channel
object, let's just tolerate their incompletely built circuits.

Patch by Tobias Pulls.

(a relay operator in #tor-relays just now was confused and thought
that dirportfrontpage could serve multiple files, like a real webserver.)

This reverts commit 5a2ab886.

This merge removes an incorrect comment about the stem version
and commit script.

And add some comments about stem

Part of 29280.

Linux is the first item in the os matrix, so we don't need to specify
it under matrix: include: .

Part of 29280.

Part of 29280.

Part of 29280.

Coverity has had trouble figuring out our csiphash implementation,
and has given spurious warnings about its behavior.

This patch changes the csiphash implementation when coverity is in
use, so that coverity can figure out that we are not about to read
beyond the provided input.

Closes ticket 31025.

Coverity doesn't understand that if begin_cell_parse() returns 0 and
sets is_begindir to 0, its address field will always be set.

Fixes bug 30126; bugfix on 0.2.4.7-alpha; Fixes CID 1447296.

Replace the 157 fallbacks originally introduced in Tor 0.3.5.6-rc
in December 2018 (of which ~122 were still functional), with a
list of 148 fallbacks (70 new, 78 existing, 79 removed) generated
in June 2019.

Closes ticket 28795.

Note that we created extra lists from DE and US, but they didn't add
any more fallbacks when we tried to merge them.