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.

Bug: T75203
Change-Id: I07736c150fe0783e09d297395ed25adf335edbd3
(cherry picked from commit bcc31a9a0fa77e91c4f3ad4a7f0e056d4bf5e713)