Change autounmountd(8) to use time_t for duration instead of double

Summary:
autounmountd(8) uses doubles to handle mount time durations. However,
it must convert to integer types, time_t in particular, to do anything
meaningful. Additionally, even though it's a floating-point value in
seconds, the sub-seconds component is never used, so it's unnecessary.

Switching type to time_t fixes an assertion on powerpc64, which checks
that a sleep value that's not -1.0 is greater than 0. On powerpc64, it
happens that the value of -1.0 gets loaded as a float (perhaps a bug in
gcc), but gets compared to a double. This compares as false, so follows
through the 'sleep != -1.0' path, and fails the assert. Since the
sub-second component isn't used in the double, just drop it and deal
with whole-integer seconds.

Reviewed by: trasz
Differential Revision: https://reviews.freebsd.org/D21109