main: add "-fsname" option

As requested in https://github.com/rfjakob/gocryptfs/issues/73 .

Documentation/MANPAGE.md

@@ -67,6 +67,11 @@ Options:
 :	Stay in the foreground instead of forking away. Implies "-nosyslog".
 	For compatability, "-f" is also accepted, but "-fg" is preferred.
 
+**-fsname string**
+:	Override the filesystem name (first column in df -T). Can also be
+	passed as "-o fsname=" and is equivalent to libfuse's option of the
+	same name. By default, CIPHERDIR is used.
+
 **-fusedebug**
 :	Enable fuse library debug output
 

cli_args.go

@@ -20,7 +20,7 @@ type argContainer struct {
 	longnames, allow_other, ro, reverse, aessiv, nonempty, raw64,
 	noprealloc bool
 	masterkey, mountpoint, cipherdir, cpuprofile, extpass,
-	memprofile, ko, passfile, ctlsock string
+	memprofile, ko, passfile, ctlsock, fsname string
 	// Configuration file name override
 	config             string
 	notifypid, scryptn int
@@ -118,6 +118,7 @@ func parseCliOpts() (args argContainer) {
 	flagSet.StringVar(&args.passfile, "passfile", "", "Read password from file")
 	flagSet.StringVar(&args.ko, "ko", "", "Pass additional options directly to the kernel, comma-separated list")
 	flagSet.StringVar(&args.ctlsock, "ctlsock", "", "Create control socket at specified path")
+	flagSet.StringVar(&args.fsname, "fsname", "", "Override the filesystem name")
 	flagSet.IntVar(&args.notifypid, "notifypid", 0, "Send USR1 to the specified process after "+
 		"successful mount - used internally for daemonization")
 	flagSet.IntVar(&args.scryptn, "scryptn", configfile.ScryptDefaultLogN, "scrypt cost parameter logN. "+

mount.go

@@ -243,7 +243,11 @@ func initFuseFrontend(key []byte, args *argContainer, confFile *configfile.ConfF
 	}
 	// Set values shown in "df -T" and friends
 	// First column, "Filesystem"
-	mOpts.Options = append(mOpts.Options, "fsname="+args.cipherdir)
+	fsname := args.cipherdir
+	if args.fsname != "" {
+		fsname = args.fsname
+	}
+	mOpts.Options = append(mOpts.Options, "fsname="+fsname)
 	// Second column, "Type", will be shown as "fuse." + Name
 	mOpts.Name = "gocryptfs"
 	if args.reverse {