Return from STRING the NTH to MTH arguments.
NTH and/or MTH can be nil, which means the last argument.
Returned arguments are separated by single spaces.
We assume whitespace separates arguments, except within quotes
and except for a space or tab that immediately follows a backslash.
Also, a run of one or more of a single character
in `comint-delimiter-argument-list' is a separate argument.
Argument 0 is the command name. (fn STRING NTH MTH)