maint: avoid new signed overflow warning on 32 bit

Prompted by http://hydra.nixos.org/build/15682577
with GCC 4.8.3 on i686

  src/tac.c:557:6: error: assuming signed overflow does not occur
  when simplifying conditional to constant [-Werror=strict-overflow]
     if (bytes_copied < 0)

This happens because copy_to_temp() is inlined in tac_nonseekable(),
thus reducing the comparison to the bytes_copied variable in
copy_to_temp.  Now this can't overflow on either 32 or 64 bit
due to the protection of the preceding fwrite().  We could use a
guard like "if (bytes_copied <= OFF_T_MAX - bytes_read)" to avoid
the warning, but rather than a runtime branch, just use an unsigned
type to avoid apparent signed overflow on systems where the accumulation
is not promoted to unsigned (32 bit size_t, 64 bit off_t).

* src/tac.c (copy_to_temp): Increment an unsigned type to
avoid the subsequent signed overflow warning.