[release-branch.go1.8] all: merge master into release-branch.go1.8

6593d865 go/ast: fix Object's doc comment about Data
c1730ae4 runtime: force workers out before checking mark roots
d10eddcb testing: make parallel t.Run safe again
2c8b70ea crypto/x509: revert SystemCertPool implementation for Windows
fcfd9185 doc/go1.8: document Plan 9 requirements
81a61a96 runtime: for plugins, don't add duplicate itabs
f674537c README.md: update and simplify
d8711919 cmd/go: fix bug help message
48d8edb5 crypto/tls: disable CBC cipher suites with SHA-256 by default
92ecd789 cmd/compile: add ZeroWB case in writebarrier
787125ab doc: 2017 is the Year of the Gopher
5b708a6b cmd/compile: lvalues are only required for == when calling runtime fns
e83d5067 vendor/golang_org/x/crypto/poly1305: revendor to pick up fix for #18673
76f981c8 net/http: skip TestServerHijackGetsBackgroundByte on Plan 9
e395e324 net/http: skip TestServerHijackGetsBackgroundByte_big on Plan 9
6a3c6c0d net/http: add another hijack-after-background-read test
467109bf all: test adjustments for the iOS builder
b2a3b54b net/http: make sure Hijack's bufio.Reader includes pre-read background byte
593ea3b3 cmd/go, misc: rework cwd handling for iOS tests
0642b8a2 syscall: export Fsid.X__val on s390x
4601eae6 doc/gdb: mention GOTRACEBACK=crash
4c4c5fc7 misc/cgo/testplugin: test that types and itabs are unique
22689c44 reflect: keep makeFuncImpl live across makeFuncStub
9cf06ed6 cmd/link: only exclude C-only symbols on darwin
9c3630f5 compress/flate: avoid large stack growth in fillDeflate
4f0aac52 cmd/go: add comment about SIGUSR2 on iOS
333f764d cmd/go, misc: switch from breakpoint to SIGUSR2
39e31d5e doc/go1.8: update timezone database version
08da8201 misc/cgo/testshared: test that types and itabs are unique
fdde7ba2 runtime: avoid clobbering C callee-save register in cgoSigtramp
f65abf6d cmd/compile: hide testdclstack behind debug flag
641ef2a7 compress/gzip: skip TestGZIPFilesHaveZeroMTimes on non-builders
0724aa81 crypto/dsa: gofmt
ac055429 net/http: deflake TestRetryIdempotentRequestsOnError
b842c9aa doc: remove inline styles

Change-Id: I642c056732fe1e8081e9d73e086e38ea0b2568cc