New settings for default max_mem_frac (0.65)

[mwibral]

Dear Developers,

I would like to suggest 0.65 as a new default fraction of the maximum available memory. This is because with ECC (e.g. only TESLA and QUADRO) only 87.5% of the total memory are available for the user. How this reported to the caller/user may vary depending on driver and opencl implementation however. Hence, it's safer to assume only 87.5% is available. Moreover, on some AMD-drivers (e.g. AMDGPU-Pro) the largest variable can only have 3/4 of the available memory. Now, 0.87*0.75~0.65. This may be a good starting point then for local testing on individual problems.