The posix_spawn attributes test is disabled. It indeed has two large issues.

One is that it assumes that SCHED_* constants are contiguous, which is not neccessary.
Another, which causes test failures, is that the test assumes that the priority ranges for all types of schedulers are same, which is not true on FreeBSD.

Patch changes test to use explicit array of SCHED_* policies constants, order of elements of the array is used to select next scheduler. The get_different_priority() is modified to work with the prio range for the new scheduler, instead of the current one.

Also, fixed several cosmetics, like double flag specification, unused MIN/MAX, use if K&R definitions etc.