NAME
        ReadItem - reads a single argument/name from command line (V36)

   SYNOPSIS
        value = ReadItem(buffer, maxchars, input)
        D0                D1        D2      D3

        long readitem(strptr, long, struct csource *)

   FUNCTION
        Reads a "word" from either input() (buffered), or via csource, if it
        is non-NULL (see <dos/rdargs.h> for more information).  handles
        quoting and some '*' substitutions (*e and *n) inside quotes (only).
        See dos/dos.h for a listing of values returned by readitem()
        (ITEM_XXXX).  A "word" is delimited by whitespace, quotes, '=', or
        an EOF.

        ReadItem always unreads the last thing read (UnGetC(fh,-1)) so the
        caller can find out what the terminator was.

   INPUTS
        buffer   - buffer to store word in.
        maxchars - size of the buffer
        input    - csource input or null (uses fgetc(input()))

   RESULT
        value - See <dos/dos.h> for return values.

   BUGS
        Doesn't actually unread the terminator.

   SEE ALSO
        readargs(), findarg(), ungetc(), fgetc(), input(), <dos/dos.h>,
        <dos/rdargs.h>, freeargs()