Fix more issues in the runtime identified using clang -Wall.

runtime/mercury_wrapper.c:
	Declare MR_usage to be a noreturn function.  This avoids
	warnings in other parts of this module.

runtime/mercury_context.c
	Only define MR_write_out_profiling_parallel_execution if
	MR_PROFILE_PARALLEL_EXECUTION_SUPPORT is defined; it is
	otherwise unused.

	Fix incorrect conversion specifiers in some calls to fprintf.

	Only define the function action_shutdown_engine and friends
	if MR_THREAD_SAFE is defined.

	Do not define MR_checking_pending_contexts if MR_HIGHLEVEL_CODE
	is defined.

runtime/mercury_debug.c:
	Protect a group of function prototypes with MR_LOWLEVEL_DEBUG.
	(The corresponding definitions were already protected by this
	macro.)

runtime/mercury_label.c:
	Delete the declaration of the function compare_entry_by_addr
	as that function is not defined anywhere.

	Delete an unused local variable.

runtime/mercury_make_type_info_body.h:
	Delete an unused local variable.

runtime/mercury_memory.c:
	Delete left over function prototypes for try_munprotect and
	explain_context.  (The functions were moved into the module
	mercury_memory_handlers a long time ago.)

runtime/mercury_memory_handlers.c:
	Protect functions that are only used in the low-level agc
	grades by the appropriate macros.

runtime/mercury_minimal.c:
runtime/mercury_mm_own_stacks.c:
	Don't define some global variables in high-level C grades that
	are not necessary in those grades.

runtime/mercury_tabling.c:
	Only define MR_table_assert_failed if MR_TABLE_DEBUG is defined
	since it is otherwise unused.