main: increase open file limit to 4096

Linux by default has a soft limit of 1024 and a hard limit of
4096 on open files. We can increase it so 4096 without root
permissions.

This should help reduce the risk of gocryptfs running out of
file descriptors, as reported at
#82 .

mount.go

@@ -147,6 +147,18 @@ func doMount(args *argContainer) int {
 		// Send SIGUSR1 to our parent
 		sendUsr1(args.notifypid)
 	}
+	// Increase the number of open files limit up to the hard limit. It's not
+	// dramatic if that fails, so do it after we have switched to syslog and not
+	// bother the user with warning.
+	var lim syscall.Rlimit
+	syscall.Getrlimit(syscall.RLIMIT_NOFILE, &lim)
+	lim.Cur = lim.Max
+	err = syscall.Setrlimit(syscall.RLIMIT_NOFILE, &lim)
+	if err != nil {
+		tlog.Warn.Printf("Setting RLIMIT_NOFILE failed: %v", err)
+	} else if lim.Cur < 4096 {
+		tlog.Warn.Printf("Low hard limit for RLIMIT_NOFILE: %d", lim.Cur)
+	}
 	// Wait for SIGINT in the background and unmount ourselves if we get it.
 	// This prevents a dangling "Transport endpoint is not connected"
 	// mountpoint if the user hits CTRL-C.