Add safe_strtoint() helper

Add safe_strtoint() helper

Adds a safe strtol helper function, modeled loosely after Wayland
scanner's strtouint.  This encapsulates the various quirks of strtol
behavior, and streamlines the interface to just handling base-10 numbers
with a simple true/false error indicator and a uint32_t return by
reference.

Test cases are loosely derived from an earlier patch by Imran Zaman.

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