Don't report the door as being in error status just because it is zero priced

See merge request !4

Even if a user is in door, the fact that it is priced as zero means it
is always reported as being in error state, and it cannot be selected by
clients from the menu. Hack around this with strcmp.

Dispense door confirmation now will only triggered in interactive shell.

See merge request !2

Added an extra confirmation step for `dispense door`

See merge request !1

- Now returns a boolean success, and takes a pointer in which to write the
  successfully parsed value (unchanged on failure)
- Also cleaned up argument processing to be flatter and slightly less hacky