[coop] Add mono_threads_suspend_policy_name

[coop] Add mono_threads_suspend_policy_name

Returns a text description of the suspend policy currently in effect.

Also combine logic that probes for suspend policy defines and environment
variables into a single function that returns an enum value; use that to drive
the individual boolean predicate functions.

The policy is subject to configure flags from when mono was built, and the
assorted environment variables at runtime:
    MONO_ENABLE_COOPERATIVE_SUSPEND
    MONO_ENABLE_COOP
    MONO_ENABLE_HYBRID_SUSPEND
(the second one is a backward-compatible alias for the first)

The logic is:

If coop xor hybrid is enabled by configure flags, always return that policy.
Otherwise if coop and hybrid env vars are set simultaneously, it's an error.
Otherwise if MONO_ENABLE_COOP or MONO_ENABLE_COOPERATIVE_SUSPEND is set use full coop.
Otherwise if MONO_ENABLE_HYBRID_SUSPEND is set use hybrid.
Otherwise use full preemptive suspend.