Vendor all dependencies (Warning: Huge changeset.)

The vendor/ folder was created with the help of @FiloSottile's gvt and
vendorcheck.

Any dependencies of Caddy plugins outside this repo are not vendored.

We do not remove any unused, vendored packages because vendorcheck -u
only checks using the current build configuration; i.e. packages that
may be imported by files toggled by build tags of other systems.

CI tests have been updated to ignore the vendor/ folder. When Go 1.9 is
released, a few of the go commands should be revised to again use ./...
as it will ignore the vendor folder by default.

vendor/github.com/codahale/aesnicheck/LICENSE

@@ -0,0 +1,21 @@
+The MIT License (MIT)
+
+Copyright (c) 2014 Coda Hale
+
+Permission is hereby granted, free of charge, to any person obtaining a copy
+of this software and associated documentation files (the "Software"), to deal
+in the Software without restriction, including without limitation the rights
+to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+copies of the Software, and to permit persons to whom the Software is
+furnished to do so, subject to the following conditions:
+
+The above copyright notice and this permission notice shall be included in
+all copies or substantial portions of the Software.
+
+THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+THE SOFTWARE.

vendor/github.com/codahale/aesnicheck/asm_amd64.s

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+// func HasAESNI() bool
+TEXT ·HasAESNI(SB),$0
+	XORQ AX, AX
+	INCL AX
+	CPUID
+	SHRQ $25, CX
+	ANDQ $1, CX
+	MOVB CX, ret+0(FP)
+	RET

vendor/github.com/codahale/aesnicheck/check_asm.go

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+// +build amd64
+
+package aesnicheck
+
+// HasAESNI returns whether AES-NI is supported by the CPU.
+func HasAESNI() bool

vendor/github.com/codahale/aesnicheck/check_generic.go

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+// +build !amd64
+
+package aesnicheck
+
+// HasAESNI returns whether AES-NI is supported by the CPU.
+func HasAESNI() bool {
+	return false
+}

vendor/github.com/codahale/aesnicheck/cmd/aesnicheck/aesnicheck.go

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+// Command aesnicheck queries the CPU for AES-NI support. If AES-NI is supported,
+// aesnicheck will print "supported" and exit with a status of 0. If AES-NI is
+// not supported, aesnicheck will print "unsupported" and exit with a status of
+// -1.
+package main
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/codahale/aesnicheck"
+)
+
+func main() {
+	if aesnicheck.HasAESNI() {
+		fmt.Println("supported")
+		os.Exit(0)
+	} else {
+		fmt.Println("unsupported")
+		os.Exit(-1)
+	}
+}

vendor/github.com/codahale/aesnicheck/docs.go

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+// Package aesnicheck provides a simple check to see if crypto/aes is using
+// AES-NI instructions or if the AES transform is being done in software. AES-NI
+// is constant-time, which makes it impervious to cache-level timing attacks. For
+// security-conscious deployments on public cloud infrastructure (Amazon EC2,
+// Google Compute Engine, Microsoft Azure, etc.) this may be critical.
+//
+// See http://eprint.iacr.org/2014/248 for details on cross-VM timing attacks on
+// AES keys.
+package aesnicheck