Ignore ECHILD in UnixEventPort::ChildSet::checkExits

If no child process has changed state, waitpid() returns zero. But if there
are no more child processes at all, it returns -1 and sets errno to ECHILD.
Since checkExits calls waitpid() in a loop, it must ignore ECHILD.

+Unit test modified to exhibit the unexpected behaviour.

cmake: drop NO_DEFAULT_PATHS when searching libraries

Add functions for async-signal-safe stringification

Alas, this means we can't use sprintf() anymore. Ugh.

Automatically reduce `Promise<RemotePromise<T>>` -> `RemotePromise<T>`.

VS 2015 is just too buggy. Working around the bugs is wasting developer time.

This change does not actually remove any of the hacks yet, so that we can undo this decision if there are significant complaints after the next release.

Just like `Promise<Promise<T>>` reduces to `Promise<T>`, we should be able to reduce `Promise<RemotePromise<T>>`. Moreover, this reduction should support pipelining, hence the reduced type should itself be `RemotePromise<T>`.

Fixes #341

This is safer for signal handlers.

This is useful especially in signal handlers where malloc() is potentially unsafe.

An inconsistency was introduced with the previous fix to allow Cap'n Proto
installations in system directories to be found by CapnProtoTargets.cmake:
if the user installed Cap'n Proto in two locations, one of which is a
system directory, e.g. /usr and /somewhere/else, and set
PKG_CONFIG_PATH=/somewhere/else/lib/pkgconfig, the dependent project
would use headers from /somewhere/else/include, but libraries from /usr/lib.

This change resolves the inconsistency, allowing PKG_CONFIG_PATH to solely
control the location of the desired installation of Cap'n Proto.

fix const-correctness on AnyPointer equality checks

Log unknown Transfer-Encoding values in kj-http

Printf debugging at its finest.

Allow .cmd/.bat files as compiler plugins on Windows.

Fixes #487
Fixes #695

Fix handling of disconnect during WebSocket::pumpTo().

Before this change, if *either* the `from` or the `to` end of the pump threw a DISCONNECTED exception, we'd then try to call `to.disconnect()`. But if the exception had been thrown from `to` in the first place, then this would throw again, this time complaining about a previous write not having completed. But the whole point was always to propagate errors from the *receiving* end to the sending end. An error on the sending end should just propagate up the stack.

Add GzipAsyncOutputStream::Decompress and ::flush

Move generic gzip output operations to private helper for easier sharing of capabilities between GzipOutputStream and GzipAsyncOutputStream.

 - Makes it possible to decompress data granularly via push-based API.
 - Adds ability to flush output at an arbitrary point.
 - Removes manual `ended` check as zlib checks it for us.

Contents of kj-gzip are guarded by KJ_HAS_ZLIB, but the library itself should be still added unconditionally to non-CAPNP_LITE build for tests to link properly.

Add limit to readAllBytes()/readAllText()

Reading an unbounded amount of data from a stream is a potential DoS vector. To manage this risk, readAllText() and readAllBytes() now accept a `limit` parameter. For source backwards-compatibility, this limit defaults to kj::maxValue.

Fix a typo

s/starp/strap/

Fix chunked body gather-write test.

The braced array literals seem to be constructed as temporaries. The test passes in debug builds, but in optimized builds, the arrays have been overwritten by the time they are used.

Chunked body fixes in kj-http

The gather-write and tryPumpFrom() member functions weren't encoding their chunk sizes in hex, and the gather-write case had an off-by-n error building the body data pieces.

Allow hash signs in path and query components in kj::Url request contexts