Use ufile_ptr for file position

This is in response to a fuzzed objcopy test that overflows file_ptr
addition.  It makes sense to use an unsigned value for current file
offset, and allows a couple of casts to be removed.

	* elf.c (assign_file_positions_for_load_sections): Make off unsigned.
	(assign_file_positions_except_relocs): Likewise.
	(_bfd_elf_assign_file_positions_for_non_load): Likewise.