thread: move get_tcr_thread() into <ukernel/thread.h>

It's a very simple function with a condition that joins with a return value
alternative. Inlining it will avoid a branch in all cases, which justifies the
micro-optimization.
exception: always require @utcb != NULL in set_pf_msg()

As per its actual use (, duh).

Also, removed some #if 0'd segments from the I/O fault handler.
thread: remove unused parameters from interrupt_ctl()

And sprinkle some more asserts wrt the I-flag in functions that disable
interrupts.