crypto: save dh params

Diffie-Hellman parameters can take a few seconds to compute,
during this time a CPU core will be fully used. This leads
to high power consumption on startup, which can be problematic
for mobile devices.

This patch introduces saving DH params to the persistent cache
to avoid regenerating them too often.
The file modification time is used to know the age of the
parameters. DH params can be reused for up to 3 days.

* Intoduce writeTime to know a file modification time
* Introduce to_wstring to convert filenames to the Windows format
* Introduce a DhParams structure to handle serialization

Tuleap: #452
Change-Id: Iaea9cf24d922fc6cfc542f8fa7b0c208ebc141d2