feature, net/tshttpproxy: pull out support for using proxies as a feature

Saves 139 KB.

Also Synology support, which I saw had its own large-ish proxy parsing
support on Linux, but support for proxies without Synology proxy
support is reasonable, so I pulled that out as its own thing.

Updates #12614

Change-Id: I22de285a3def7be77fdcf23e2bec7c83c9655593
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

feature/feature.go

@@ -50,7 +50,8 @@ func (h *Hook[Func]) Set(f Func) {
 }
 
 // Get returns the hook function, or panics if it hasn't been set.
-// Use IsSet to check if it's been set.
+// Use IsSet to check if it's been set, or use GetOrNil if you're
+// okay with a nil return value.
 func (h *Hook[Func]) Get() Func {
 	if !h.ok {
 		panic("Get on unset feature hook, without IsSet")
@@ -64,6 +65,11 @@ func (h *Hook[Func]) GetOk() (f Func, ok bool) {
 	return h.f, h.ok
 }
 
+// GetOrNil returns the hook function or nil if it hasn't been set.
+func (h *Hook[Func]) GetOrNil() Func {
+	return h.f
+}
+
 // Hooks is a slice of funcs.
 //
 // As opposed to a single Hook, this is meant to be used when