Factor new vfs module out of cmd/mountlib

This is an OS style file system abstraction with directory caching
used in mount, cmount, serve webdav and serve http.

vfs/vfs_unix.go

@@ -0,0 +1,19 @@
+// +build linux darwin freebsd
+
+package vfs
+
+import (
+	"github.com/spf13/pflag"
+	"golang.org/x/sys/unix"
+)
+
+// add any extra platform specific flags
+func platformFlags(flags *pflag.FlagSet) {
+	flags.IntVarP(&Umask, "umask", "", Umask, "Override the permission bits set by the filesystem.")
+	Umask = unix.Umask(0) // read the umask
+	unix.Umask(Umask)     // set it back to what it was
+	UID = uint32(unix.Geteuid())
+	GID = uint32(unix.Getegid())
+	flags.Uint32VarP(&UID, "uid", "", UID, "Override the uid field set by the filesystem.")
+	flags.Uint32VarP(&GID, "gid", "", GID, "Override the gid field set by the filesystem.")
+}