This changes all of the builtins to behave like `string` to return
STATUS_INVALID_ARGS (121) if the args passed to the command don't make
sense. Also change several of the builtins to use the existing symbols
(e.g., STATUS_CMD_OK and STATUS_CMD_ERROR) rather than hardcoded "0"
and "1" for consistency and to make it easier to find such values in
the future.
Fixes#3985
Previously 'set -ql' would only look for variables in the
immediate local scope. This was not very useful. It's also
arguably surprising, since a 'set -l' in a function, followed
by a 'set -ql' in a child block, would fail. There was also no
way to check for a function-scoped variable, since omitting the
scope would also pull in global variables.
We could revisit this and introduce an explicit function scope.
Fixes#2502
