support 64-bit time_t values

[dun]

The credential format v3 stores the time_t at which the credential is encoded as a 32-bit value. This makes it vulnerable to the Epochalypse (aka the Year 2038 problem).

As such, the planned extensible credential format (#87) and libmunge API should accommodate 64-bit time_t values.

• LWN: Approaching the kernel year-2038 end game
• LWN: Debian discusses how to handle 2038
• 64-bit Time on Debian
• 64-bit Time on OpenBSD
• OpenBSD 5.5 Release Notes
• Year 2038 problem