Explicitly disabling 'unix-abstract:' on non-Linux Unix platforms
needs more work than this, but maybe it's not worth it since the
result would just be a different runtime failure message. For now
just be consistent

Abstract unix socket addresses can now be specified with the
'unix-abstract:' prefix, and the 'unix:' prefix now explicitly
rejects addresses which contain NULL bytes

Since kj::StringPtr supports embedded NULs, copy its entire data
buffer to the sockaddr_un structure instead of using strcpy.
On Linux, this allows the use of the abstract socket namespace.

It turns out this is more than just android. Some Debian Stretch docker image I'm working with seems to have this problem (but Stretch in general does not??).

Fix link errors with -fsanitize=vptr

Add new `capnp convert` command with JSON support.

This supersedes the `capnp encode` and `capnp decode` commands. It didn't make sense to add JSON to those commands since it was unclear if JSON should be thought of as the "encoded" or "decoded" format. `convert` allows mapping anything to anything.

This command is also useful for, say, converting unpacked format to packed format or vice versa, which can now be done without a schema.

Allow an ExceptionCallback to control how new threads' ExceptionCallbacks are initialized.

Fix minor typo: varibale -> variable

Clarify RPC spec documentation based on mail thread

Kenton rightly pointed out that this is an unnecessary round-trip.

Update README.md

protobuf link update

Currently only implemented for Linux (when using futexes), since that's what I use. Will expand to other platforms later.

synchrozines behavior with autoconf build

https://github.com/capnproto/capnproto/issues/515

https://groups.google.com/d/topic/capnproto/_1kBtRSC51s/discussion

Doc: Enforce comment-after-code rule

Install capnp-json.pc

[ 66%] Linking CXX executable capnpc-c++
cd /home/edward.catmur/build/capnproto@master/c++/src/capnp && /usr/local/bin/cmake -E cmake_link_script CMakeFiles/capnpc_cpp.dir/link.txt --verbose=1
/usr/lib64/icecc/bin/c++   -fsanitize=vptr   CMakeFiles/capnpc_cpp.dir/compiler/capnpc-c++.c++.o  -o capnpc-c++ -rdynamic libcapnp.a ../kj/libkj.a -lpthread
libcapnp.a(layout.c++.o):(.data+0x38): undefined reference to `typeinfo for capnp::ClientHook'
libcapnp.a(layout.c++.o):(.data+0x3b8): undefined reference to `typeinfo for capnp::ClientHook'
collect2: error: ld returned 1 exit status

$ g++ --version
g++ (GCC) 5.4.0
Copyright (C) 2015 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

While comments should appear after the code, some examples violate the convention. This commit enforces the rule.

Minor: fix typo in `capnp help encode` documentation