Finish up custom option implementation.

Finish up custom option implementation.

cups/mark.c:
    - ppdMarkOption(): Support option choices starting with "{",
      e.g. "{param1=value param2=value}".

cups/ppd.c:
    - ppdClose(): Don't free min/max string values.
    - ppdOpen2(): Min/max limits for strings are now integers.

cups/ppd.h:
    - Add ppd_cplimit_u and ppd_cplimit_t types, and use them for
      the minimum and maximum values for a custom parameter.

git-svn-id: svn+ssh://src.apple.com/svn/cups/cups.org/trunk@5190 7a7537e8-13f0-0310-91df-b6672ffda945