A couple small lockserver client improvements.

1. Previously, typing ` `, `lock`, or `unlock` into a lockserver
   client's REPL would cause the client to crash because of some
   `strtok` calls that were returning `NULL`. Now, this bug is fixed;
   `strtok` returns are checked to be `NULL`, and no input should crash
   the client.
2. Previously, typing in an unrecognized command into the lockserver
   client's repl would print `Unknown command.. Try again!`. Now, it
   also prints out the set of legal commands `Usage: exit | q | lock
   <key> | unlock <key>`. This makes it a bit easier for someone
   tinkering around with TAPIR for the first time (like me) to know what
   to type.