mach-o: remove name_len field.

bfd/
	* mach-o.h (bfd_mach_o_dylinker_command)
	(bfd_mach_o_dylib_command, bfd_mach_o_fvmlib_command): Remove
	name_len field.
	* mach-o.c (bfd_mach_o_read_dylinker, bfd_mach_o_read_dylib)
	(bfd_mach_o_read_fvmlib): Adjust after name_len removal.
mach-o: Define copy_private_header instead of copy_private_bfd.

bfd/
	* mach-o-target.c (bfd_mach_o_bfd_copy_private_header_data):
	Define instead of bfd_mach_o_bfd_copy_private_bfd_data.
	* mach-o.c (bfd_mach_o_bfd_copy_private_bfd_data): Rename.
	* mach-o.h (bfd_mach_o_bfd_copy_private_bfd_data): Likewise.