Standardize error checking for strtol calls

Standardize error checking for strtol calls

This tightens up the strtol() error checking in several places where it
is used for parsing environment variables, and in the backlight
interface that is reading numbers from files under /sys/class/backlight.
All of these uses are expecting strings containing decimal numbers and
nothing else, so the error checking can all be tightened up and made
consistent with other strtol() calls.

This follows the error checking style used in Wayland
(c.f. wayland-client.c and scanner.c) and c.f. commit cbc05378.

Signed-off-by: Bryce Harrington <bryce@osg.samsung.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Peter Hutterer <peter.hutterer@who-t.net>