Adds new version of help50

[dmalan]

To be squashed when merging.

This new version is implemented in Bash (instead of Python) as follows, wherein usage is inspired by systemctl, even though it doesn't run as a daemon but, rather, per login shell. It runs locally and automatically now, without any server.

• help50 start sets $HELP50 to $$, the PID of the shell in which the command was run and launches script, which logs standard I/O to /tmp/help50.$$.typescript.
• help50 stop sends SIGHUP to the PID of script.
• help50 status checks for $HELP50, which is set only when help50 is started for a shell.
• help50 disable writes /tmp/help50.lock.
• help50 enable deletes /tmp/help50.lock.
• help50 is-enabled checks for /tmp/help50.lock.

---

• /etc/profile.d/help50.sh is a config that that's only sourced when $HELP50 is set.
  • It sets $PROMPT_COMMAND to _help50, which is a Bash function implemented therein that, if the most recent command exited with non-0 status (per $?), checks for /tmp/help50.$HELP50.typescript, passes it as standard input to each executable in /opt/cs50/lib/help50/ (implemented in any language).
  • If any of those echo output, it passes it to a _helpful function that, by default, displays it in yellow to help the user.
  • It none of those echo output, it passes the failed command's typescript to _helpless instead, which doesn't do anything in cs50/cli but can be overridden in cs50/codespace to relay it to ddb50.
  • Else if the most recent command exited successfully, _helped is called, which doesn't do anything in cs50/cli but can be overridden in cs50/codespace to indicate to the user that help is (no longer) available.
• /etc/profile.d/cli.sh starts help50 automatically.
• /opt/cs50/lib/cli contains several helper functions (written in Bash) that our own wrappers and help50 use.

---

$ git fetch
$ git checkout help50
$ make build
$ make run

/mnt/ $ help50 start

/mnt/ $ ps f
  PID TTY      STAT   TIME COMMAND
    1 pts/0    Ss     0:00 bash --login
   24 pts/0    S      0:00 /bin/bash /opt/cs50/bin/help50 start
   26 pts/0    S+     0:00  \_ script --append --command bash --login ; exit 1 --flush --quiet --return /tmp/help50.1.typescript
   27 pts/1    Ss     0:00      \_ sh -c bash --login ; exit 1
   28 pts/1    S      0:00          \_ bash --login
  143 pts/1    R+     0:00              \_ ps f

/mnt/ $ help50 stop

Session terminated, killing shell... ...killed.

/mnt/ $ ps f
  PID TTY      STAT   TIME COMMAND
    1 pts/0    Ss     0:00 bash --login
  178 pts/0    R+     0:00 ps f

[dmalan]

Thank you! Addressed all of these and working on a few more tweaks before merging!