cmake: check and store the size of mode_t
Make KEY_MODE work when mode_t is not the same as int

Even though the file modes passed using KEY_MODE should be mode_t,
they really get through as integer, thus picking them from the varargs
as mode_t causes troubles when sizeof(mode_t) != sizeof(int). Hence,
always take them out as int from the varargs.

Adapt test_meta to test large values for the mode only when
sizeof(mode_t) > 2.
Merge pull request #67 from pinotree/mode_t

Fix KEY_MODE vararg when mode_t is not int