util/cutils: Clean up variable names around qemu_strtol()

Name same things the same, different things differently.

* qemu_strtol()'s parameter @nptr is called @p in
  check_strtox_error().  Rename the latter.

* qemu_strtol()'s parameter @endptr is called @next in
  check_strtox_error().  Rename the latter.

* qemu_strtol()'s variable @p is called @endptr in
  check_strtox_error().  Rename both to @ep.

* qemu_strtol()'s variable @err is *negative* errno,
  check_strtox_error()'s parameter @err is *positive*.  Rename the
  latter to @libc_errno.

Same for qemu_strtoul(), qemu_strtoi64(), qemu_strtou64(), of course.

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487708048-2131-8-git-send-email-armbru@redhat.com>
