timeval: Initialize 'unix_epoch' for Windows correctly.

Till now, we were initializing 'unix_epoch' through time_init().
But if there was a call directly to xgettimeofday(), we would
miss the initialization causing overflows. This commit fixes it
by pre-calculating the value and assigning it globally.

Also add-in a missing return statement.

Reported-by: Alessandro Pilotti <apilotti@cloudbasesolutions.com>
Reported-by: Linda Sun <lsun@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>