wn_gets -- get string with validation

        USAGE

          (char *) wn_gets(wn, buf, va, uva)
          WINDOWPTR wn;
          char *buf;
          int va;
          char *uva;

                wn -    window handle
                buf -   user buffer for string
                va -    input validation to be used
                uva -   user validation list [optional]

          va specifies the type of input validation to be performed as
          data is being entered. Options are:

                (1) none                no restrictions - accept everything
                (2) integer             accept: 0 thru 9 + -
                (3) floating point      accept: 0 thru 9 + - .
                (4) alpha only          accept: a thru z (upper & lower case)
                (5) upper case only     accept: A thru Z
                (6) validation list     accept: only those characters
                    (optional)                  specified in the uva string.

                ORing va with 0x8000 disables data entry character echo.

          The following editing functions are supported:

                . backspace & rubout do the logical things
                . , X, and  ipe the field clean
                . Return and Esc end the input function

          Data entry takes place at the current logical cursor location.
          You  can, of course, position the cursor to where you wish
          prior to calling wn_gets.

          Example:

                wn_printf(wn,"Enter your name > ");
                wn_get(wn,buf,4,0);

        RETURNS

          Pointer to buf or NULL if error

        CAUTIONS

          The window "wn" automatically becomes the top window tile upon
          return.