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.

caddyhttp/httpserver/siteconfig.go

@@ -38,8 +38,8 @@ type SiteConfig struct {
 	// for a request.
 	HiddenFiles []string
 
-	// Max amount of bytes a request can send on a given path
-	MaxRequestBodySizes []PathLimit
+	// Max request's header/body size
+	Limits Limits
 
 	// The path to the Caddyfile used to generate this site config
 	originCaddyfile string
@@ -71,6 +71,12 @@ type Timeouts struct {
 	IdleTimeoutSet       bool
 }
 
+// Limits specify size limit of request's header and body.
+type Limits struct {
+	MaxRequestHeaderSize int64
+	MaxRequestBodySizes  []PathLimit
+}
+
 // PathLimit is a mapping from a site's path to its corresponding
 // maximum request body size (in bytes)
 type PathLimit struct {