net/http: make Client.Timeout return net.Error errors indicating timeout

Co-hacking with Dave Cheney.

Fixes #9405

Change-Id: I14fc3b6a47dcdb5e514e93d062b804bb24e89f47
Reviewed-on: https://go-review.googlesource.com/1875
Reviewed-by: Dave Cheney <dave@cheney.net>
image/draw: fix crash in clip

Fixes #9177

Change-Id: I1c7e57f0f0a9b00fb3ddc7fa4844ac53ea6df46f
Reviewed-on: https://go-review.googlesource.com/1876
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
runtime: fix brk_ signature for plan9

with uintptr, the check for < 0 will never succeed in mem_plan9.go's
sbrk() because the brk_ syscall returns -1 on failure. fixes the plan9/amd64 build.

this failed on plan9/amd64 because of the attempt to allocate 136GB in mallocinit(),
which failed. it was just by chance that on plan9/386 allocations never failed.

Change-Id: Ia3059cf5eb752e20d9e60c9619e591b80e8fb03c
Reviewed-on: https://go-review.googlesource.com/1590
Reviewed-by: Anthony Martin <ality@pbrane.org>
Reviewed-by: David du Colombier <0intro@gmail.com>
Reviewed-by: Aram Hăvărneanu <aram@mgk.ro>
net/http: Fix Range off-by-one error

Given a file of size N, a request for "Range: bytes=N-*" should
return a 416 [1].  Currently, it returns a 206 and a body of 0
bytes, with the illegal Content-Range of "bytes N-(N-1)/N" [2].

[1]: RFC 7233, sec 2.1: "If a valid byte-range-set includes at least one
byte-range-spec with a first-byte-pos that is less than the current
length of the representation, [...]".  sec 3.1: "If all of the
preconditions are true, the server supports the Range header field for
the target resource, and the specified range(s) are invalid or
unsatisfiable, the server SHOULD send a 416 (Range Not Satisfiable)
response."

[2]: RFC 7233, sec 4.2: "A Content-Range field value is invalid if it
contains a byte-range-resp that has a last-byte-pos value less than its
first-byte-pos value, [...]"

Fixes #8988

Change-Id: If3e1134e7815f5d361efea01873b29aafe3de817
Reviewed-on: https://go-review.googlesource.com/1862
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
flag: Some idiomatic fixes

Make golint a bit happier

Change-Id: I8a14342f3e492e92bf5efa611f9ef91176624031
Reviewed-on: https://go-review.googlesource.com/1891
Reviewed-by: Minux Ma <minux@golang.org>