Prepare linux_find_memory_regions_full & co. for move

Prepare linux_find_memory_regions_full & co. for move

	* linux-tdep.c (linux_find_memory_region_ftype): Comment.
	(linux_find_memory_regions_full): Change signature and prepare
	for moving to linux-maps.
	(linux_find_memory_regions_data): Rename field 'obfd' to 'data'.
	(linux_find_memory_regions_thunk): New.
	(linux_find_memory_regions_thunk): Use 'data' field instead of 'obfd'.
	(linux_find_memory_regions_gdb): New.
	(linux_find_memory_regions): Rename argument 'obfd' to 'func_data'.
	(linux_make_mappings_corefile_notes): Use
	linux_find_memory_regions_gdb.
	* target.c (read_alloc_pread_ftype): New typedef.
	(target_fileio_read_alloc_1_pread): New function.
	(read_alloc): Refactor from target_fileio_read_alloc_1.
	(read_stralloc_func_ftype): New typedef.
	(target_fileio_read_alloc_1): New implementation. Use read_alloc.
	(read_stralloc): Refactored from target_fileio_read_stralloc.
	(target_fileio_read_stralloc): New implementation, use read_stralloc.