mach-o.c: use boolean instead of int to return status.

bfd/
	* mach-o.c (bfd_mach_o_write_thread)
	(bfd_mach_o_write_section_32, bfd_mach_o_write_section_64)
	(bfd_mach_o_write_segment_32, bfd_mach_o_write_segment_64)
	(bfd_mach_o_read_dylinker, bfd_mach_o_read_dylib)
	(bfd_mach_o_read_prebound_dylib, bfd_mach_o_read_prebind_cksum)
	(bfd_mach_o_read_twolevel_hints, bfd_mach_o_read_fvmlib)
	(bfd_mach_o_read_thread, bfd_mach_o_read_dysymtab)
	(bfd_mach_o_read_symtab, bfd_mach_o_read_uuid)
	(bfd_mach_o_read_linkedit, bfd_mach_o_read_str)
	(bfd_mach_o_read_dyld_info, bfd_mach_o_read_segment)
	(bfd_mach_o_read_segment_32, bfd_mach_o_read_segment_64)
	(bfd_mach_o_read_command): Now return a boolean status.
	Adjust return statements.
	(bfd_mach_o_write_contents, bfd_mach_o_scan): Adjust tests.
	(bfd_mach_o_core_file_failing_command): Remove useless initialization.