Allow passing arbitrary options to fusermount

Extend the FUSE mount options to support passing a collection of textual
options that are directly passed to fusermount.

Fixes buildbarn/bb-clientd#6.

Author: jmmv

pkg/filesystem/virtual/configuration/fuse_mount_enabled.go

@@ -72,6 +72,7 @@ func (m *fuseMount) Expose(terminationGroup program.Group, rootDirectory virtual
 			FsName:      m.fsName,
 			AllowOther:  m.configuration.AllowOther,
 			DirectMount: m.configuration.DirectMount,
+			Options:     m.configuration.Options,
 		})
 	if err != nil {
 		return util.StatusWrap(err, "Failed to create FUSE server")

pkg/proto/configuration/filesystem/virtual/virtual.pb.go

@@ -133,6 +133,9 @@ message FUSEMountConfiguration {
   //
   // Recommended value: unset
   map<string, string> linux_backing_dev_info_tunables = 9;
+
+  // List of options to pass directly to fusermount.
+  repeated string options = 10;
 }
 
 message NFSv4MountConfiguration {