libbacklight: Fix backlight never gets initialized

In 913d7c15 stricter error checking was
introduced to the strtol call, which broke reading backlight values.
Since every sysfs backlight file ends with a newline.

As noted in a comment in the previous MR to prevent damaged pointers
after calling asprintf, replace every asprintf call with str_printf.

Previous-MR: wayland/weston!543



Signed-off-by: Sören Meier <soerenmeier@livgood.ch>