Use system default location for cafile when using php fopen.
Up to 5.5, PHP does not accept any certificates if cafile/capath
is not set. (From 5.6 it uses the system default CA budle, which is
going to be a better choice than anything we can guess.) So try
to guess the location of the system default CA bundle.
Won't work on windows, but that's a lost cause anyway because PHP
(pre-5.6) can't handle the windows CA file format.