proc,proc/native,proc/gdbserial: initial plugin support (#1413)

Adds initial support for plugins, this is only the code needed to keep
track of loaded plugins on linux (both native and gdbserial backend).

It does not actually implement support for debugging plugins on linux.

Updates #865

service/api/conversions.go

@@ -314,3 +314,7 @@ func ConvertRegisters(in []proc.Register) (out []Register) {
 func ConvertCheckpoint(in proc.Checkpoint) (out Checkpoint) {
 	return Checkpoint(in)
 }
+
+func ConvertImage(image *proc.Image) Image {
+	return Image{Path: image.Path}
+}