Ninja is not installed by default on the macOS machines; stop trying to
use it.  Instead use `make -j` which should be roughly equivalent in
performance but supported everywhere.

(And move the ci scripts into the `ci` directory.)

Since we have migrated to Azure Pipelines, we have deprecated and
subsequentally removed all infrastructure for AppVeyor and
Travis. Thus it doesn't make a lot of sense to have the split
between "ci/" and "azure-pipelines/" directories anymoer, as
"azure-pipelines/" is essentially our only CI.

Move all CI scripts into the "azure-pipelines/" directory to have
everything centrally located and to remove clutter in the
top-level directory.

Subtle changes in the host OS can have impacts in the CI system that
may be hard to debug.  We previously showed the results of `uname` which
can be difficult to interpret.  Provide more information where
available.

This reverts commit a2d73f5643814cddf90d5bf489332e14ada89ab8.
Using clar to propagate the XML settings was a mistake.

During the conversion of our CI scripts in bf418f09c (ci: refactor unix
ci build/test scripts, 2018-07-14), we accidentally dropped the
"-DENABLE_WERROR=ON" switch in our cmake invocation. Re-add it to help
us catch compiler warnings early.

This change adds support for building a fuzz target for exercising the
packfile parser, as well as documentation. It also runs the fuzzers in
Travis to avoid regressions.