eloop: Allow for for timeouts greater than time_t and time wrapping

eloop: Allow for for timeouts greater than time_t and time wrapping

On some systems time_t is int32_t.
However, on wire timeouts are generally uint32_t.
As such, we need to avoid using timespec internally except to
record when the timeout was created. Instead, record the timeout
as unsigned int seconds and long nanoseconds.

On long running systems using time_t as int32_t, monotonic time
would wrap after approximately 68 years. It's highly unlikely dhcpcd
would be running for so long, but just incase it does, the code
should now cope.