all: Use context in lib/dialer (#6177)

* all: Use context in lib/dialer

* a bit slimmer

* https://github.com/syncthing/syncthing/pull/5753

* bot

* missed adding debug.go

* errors.Cause

* simultaneous dialing

* anti-leak

lib/rc/rc.go

@@ -166,7 +166,7 @@ func (p *Process) Get(path string) ([]byte, error) {
 	client := &http.Client{
 		Timeout: 30 * time.Second,
 		Transport: &http.Transport{
-			Dial:              dialer.Dial,
+			DialContext:       dialer.DialContext,
 			Proxy:             http.ProxyFromEnvironment,
 			DisableKeepAlives: true,
 		},