Get rid of VEC(mem_range_s)

Get rid of VEC(mem_range_s)

This patch replaces the last usages of VEC(mem_range_s) with
std::vector<mem_range>.  This allows getting rid of a few cleanups and
of the DEF_VEC_O(mem_range_s).

I added a test for normalize_mem_ranges to make sure I didn't break
anything there.

Regtested on the buildbot.

gdb/ChangeLog:

	* memrange.h (struct mem_range): Define operator< and operator==.
	(mem_range_s): Remove.
	(DEF_VEC_O (mem_range_s)): Remove.
	(normalize_mem_ranges): Change parameter type to std::vector.
	* memrange.c (compare_mem_ranges): Remove.
	(normalize_mem_ranges): Change parameter type to std::vector,
	adjust to vector change.
	* exec.c (section_table_available_memory): Return vector, remove
	parameter.
	(section_table_read_available_memory): Adjust to std::vector
	change.
	* remote.c (remote_read_bytes): Adjust to std::vector
	change.
	* tracepoint.h (traceframe_available_memory): Change parameter
	type to std::vector.
	* tracepoint.c (traceframe_available_memory): Change parameter
	type to std::vector, adjust.
	* gdb/mi/mi-main.c (mi_cmd_trace_frame_collected): Adjust to
	std::vector change.
	* gdb/Makefile.in (SUBDIR_UNITTESTS_SRCS): Add
	unittests/memrange-selftests.c.
	(SUBDIR_UNITTESTS_OBS): Add memrange-selftests.o.
	* gdb/unittests/memrange-selftests.c: New file.