Remove references to obsolete isCallersFault

Misspelt macro name in undef

It looks like it's pretty normal for browsers to do this.

The entity-body would never be marked "done", breaking the pipeline for subsequent requests/responses.

(In practice `Content-Length: 0` is rare since normally only GET requests don't have content and they don't pass `Content-Length` at all.)

At some point isCallersFault was marked as obsolete, but the description
of abort still referenced it, suggesting particular usage. This patch
removes said references.

Since `git-diff` does *such* a good job with prose: This just removes
the two parentheticals and re-flows the paragraph.

or, the world's least significant pull request.

kj::Directory::list*() should return sorted lists to avoid non-determinism.

fix typos: complied -> compiled

* Announce 0.6 release on blog, and update site.

* When pushing capnproto-next site, include future blog posts.

* Deeper headings.

* Linkify people's names to Github profiles, and finish sentence about JSON.

* Link to exact second in JGC video.

Fix VS2017 ICE and add build matrix to appveyor.yml

Update outdated comment, and require CMake 3.1, which allows us to take
advantage of target_compile_features.

Projects which use Cap'n Proto require C++11, because they must be able to
compile our headers. Old gcc defaults to C++98, forcing users to manually
specify the -std=gnu++11 flag in their CMake projects. CMake 3.1
introduced the target_compile_features command which removes this
necessity by automatically communicating the C++ standard level
requirement to client projects.

Specifically, if target `kj` requires a C++11 feature publicly, then all
targets which link to `kj` will also require that C++11 feature, and get
the -std=gnu++11 flag for free. If that target is a library, such as
`kj-async`, and `kj-async` links publicly to `kj`, the requirement is also
transitive to all targets which link to `kj-async`.

Note that CMake's default behavior is to request compiler-specific
extensions, such as those provided by the -std=gnu++11 flag. You must
specifically opt out of these extensions. I'm not aware of any way to
propagate this nuance in CMake, but added a check to at least prevent
users from compiling Cap'n Proto without extensions.

With this change, the c++/samples/CMakeLists.txt will build with MinGW
with no change beyond requiring a later version of CMake.

This will break out MinGW, VS2015, and VS2017 builds into separate jobs.
While they will still probably build in serial, we'll at least get better
granularity on test results.

VS2015 only barely compiled ~NullableValue's noexcept specifier
expression. VS2017 can't at all, so we're going with noexcept(false) for
MSVC for now.

KJ Filesystem API

This reverts commit f836a5fc.