Support registering arbitrary file descriptors with the runtime poller

[DemiMarie]

Please answer these questions before submitting your issue. Thanks!

What version of Go are you using (go version)?

go version go1.8.1 linux/amd64

What operating system and processor architecture are you using (go env)?

GOARCH="amd64"
GOBIN=""
GOEXE=""
GOHOSTARCH="amd64"
GOHOSTOS="linux"
GOOS="linux"
GOPATH="/home/dobenour/go"
GORACE=""
GOROOT="/home/dobenour/archives/go"
GOTOOLDIR="/home/dobenour/archives/go/pkg/tool/linux_amd64"
GCCGO="gccgo"
CC="gcc"
GOGCCFLAGS="-fPIC -m64 -pthread -fmessage-length=0 -fdebug-prefix-map=/tmp/go-build801431881=/tmp/go-build -gno-record-gcc-switches"
CXX="g++"
CGO_ENABLED="1"
PKG_CONFIG="pkg-config"
CGO_CFLAGS="-g -O2"
CGO_CPPFLAGS=""
CGO_CXXFLAGS="-g -O2"
CGO_FFLAGS="-g -O2"
CGO_LDFLAGS="-g -O2"

What did you do?

Looked for a way to register an externally-provided Fd with the runtime poller.

What did you expect to see?

Such an API exists

What did you see instead?

No such API.

In general, file descriptors can appear from sources outside of the standard library (such as C libraries). So it should be possible to register them with the runtime poller.

[bradfitz]

Dup of some of:

#6817.
#7903.
#15021.
#18507.
#19093.
#19098.

/cc @ianlancetaylor