swarm/api: FUSE read-write support (#13872)

- Moved fuse related code in a new package, swarm/fuse
- Added write support
  - Create new files
  - Delete existing files
  - Append to files (with limitations)
- More test coverage

swarm/fuse/swarmfs_fallback.go

@@ -0,0 +1,51 @@
+// Copyright 2017 The go-ethereum Authors
+// This file is part of the go-ethereum library.
+//
+// The go-ethereum library is free software: you can redistribute it and/or modify
+// it under the terms of the GNU Lesser General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+//
+// The go-ethereum library is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the
+// GNU Lesser General Public License for more details.
+//
+// You should have received a copy of the GNU Lesser General Public License
+// along with the go-ethereum library. If not, see <http://www.gnu.org/licenses/>.
+
+// +build !linux,!darwin,!freebsd
+
+package fuse
+
+import (
+	"errors"
+)
+
+var errNoFUSE = errors.New("FUSE is not supported on this platform")
+
+func isFUSEUnsupportedError(err error) bool {
+	return err == errNoFUSE
+}
+
+type MountInfo struct {
+	MountPoint     string
+	StartManifest  string
+	LatestManifest string
+}
+
+func (self *SwarmFS) Mount(mhash, mountpoint string) (*MountInfo, error) {
+	return nil, errNoFUSE
+}
+
+func (self *SwarmFS) Unmount(mountpoint string) (bool, error) {
+	return false, errNoFUSE
+}
+
+func (self *SwarmFS) Listmounts() ([]*MountInfo, error) {
+	return nil, errNoFUSE
+}
+
+func (self *SwarmFS) Stop() error {
+	return nil
+}