lights: Cleanup read/write API

 * Use O_WRONLY for write
 * Return errno for write
 * Log all failures
 * Fix read_int():
   We would always treat strtol(...) = 0 as an error,
   although the real value can very well be 0.
   Check the end pointer to fix this.

Change-Id: Id2bc9acc103a7a6400dd06f3904af37cbb0b5174