rfc14: need a way to specify max duration in jobspec

[grondo]

The RFC 14 definition of the duration attribute is currently:

duration

The value of the duration attribute is a floating-point number greater than or equal to zero representing
time span in seconds. If duration is greater than zero, then the scheduler SHALL allocate the requested
resources for the number of seconds specified in duration. A duration value of 0. SHALL be interpreted as
unlimited or unset, and the scheduler SHALL use a duration that is the minimum of any configured duration
default and the time span to the currently defined instance expiration. If no default duration is configured,
and the current instance has no expiration, then resources shall be allocated without expiration.

As noted by @ryanday36 in #332, a way to specify that a job should get the maximum queue time limit instead of the default time limit would be a nice feature. One idea would be to use something like the maximum positive integer that can be represented as 32 bit floating-point (2^24? Or a little over 6 months). Another idea would be to relax the requirement that duration be positive and allow -1. to represent a request for the maximum allowable duration.

Either way, frontend tools could offer a special option or -t max and substitute whatever value we decide upon.