add support for build tag to allow to disable some features

The following build tags are available:

- "nogcs", disable Google Cloud Storage backend
- "nos3", disable S3 Compabible Object Storage backends
- "nobolt", disable Bolt data provider
- "nomysql", disable MySQL data provider
- "nopgsql", disable PostgreSQL data provider
- "nosqlite", disable SQLite data provider
- "noportable", disable portable mode

cmd/portable_disabled.go

@@ -0,0 +1,9 @@
+// +build noportable
+
+package cmd
+
+import "github.com/drakkan/sftpgo/utils"
+
+func init() {
+	utils.AddFeature("-portable")
+}