all: merge master into release-branch.go1.7

7a622740 net/http: make Transport use new connection if over HTTP/2 concurrency limit
219ca602 doc: fix required OS X version inconsistency for binary downloads
26015b95 runtime: make stack 16-byte aligned for external code in _rt0_amd64_linux_lib
9fde86b0 runtime, syscall: fix kernel gettimeofday ABI change on iOS 10
3a03e877 os: check for waitid returning ENOSYS
10316757 net/http: update bundled http2 for flow control window adjustment fix
da070bed syscall: fix Gettimeofday on macOS Sierra
f135c326 runtime: initialize hash algs before typemap

Change-Id: Ie176f3db1e253d75ae8e56b16d3fd9900b37dde3