objcopy: Avoid shadowing optarg.

objcopy: Avoid shadowing optarg.

In commit 7173b38a442c007a554ea200817a0eadce89c87b I used optarg as the
name for a function parameter, shadowing the global of that name.  This
commit changes the function parameter to be called arg.

binutils/ChangeLog:

	* objcopy.c (init_section_add): Rename optarg to arg in order to
	avoid shadowing a global variable.