Limited to 6 chars, yet some values are as long as 12 chars.

Also, eliminates this warning from RELEASE builds (only) using gcc version
12.2.0 (Debian 12.2.0-14):
js_uifc.c: In function ‘js_uifc_init’:
sbbs.h:230:56: warning: writing 8 bytes into a region of size 7 [-Wstringop-overflow=]
  230 |                                         (ret)[JSSTSpos]=(char)JSSTSstrval[JSSTSpos]; \
      |                                         ~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~
sbbs.h:242:9: note: in expansion of macro ‘JSSTRING_TO_ASTRING’
  242 |         JSSTRING_TO_ASTRING((cx), JSVTSstr, (ret), (maxsize), (lenptr)); \
      |         ^~~~~~~~~~~~~~~~~~~
js_uifc.c:609:17: note: in expansion of macro ‘JSVALUE_TO_ASTRING’
  609 |                 JSVALUE_TO_ASTRING(cx, argv[1], mode, 7, NULL);
      |                 ^~~~~~~~~~~~~~~~~~
In file included from sbbs.h:255:
/home/rswindell/sbbs/src/sbbs3/../xpdev/genwrap.h:77:20: note: destination object of size 7 allocated by ‘__builtin_alloca’
   77 | #define alloca(sz) __builtin_alloca(sz)
      |                    ^~~~~~~~~~~~~~~~~~~~
sbbs.h:228:43: note: in expansion of macro ‘alloca’
  228 |                         if(((ret)=(char *)alloca((*JSSTSlenptr)+1))) { \
      |                                           ^~~~~~
sbbs.h:242:9: note: in expansion of macro ‘JSSTRING_TO_ASTRING’
  242 |         JSSTRING_TO_ASTRING((cx), JSVTSstr, (ret), (maxsize), (lenptr)); \
      |         ^~~~~~~~~~~~~~~~~~~
js_uifc.c:609:17: note: in expansion of macro ‘JSVALUE_TO_ASTRING’
  609 |                 JSVALUE_TO_ASTRING(cx, argv[1], mode, 7, NULL);
      |                 ^~~~~~~~~~~~~~~~~~