Fix security, tighter str comparison, fix warnings

 * When reading our and the caller's path, we were mistakingly reading only the
   first sizeof(char*) characters, so anything that was running from a path
   starting with '/sys' was being considered to be us
 * strcmp'ing strings instead of comparing only for the length of the first of
   them. Avoids possible executions from /system/xbin/suwhatever .
 * Fixed several compile-time warnings

Change-Id: I2af07f7bf5078acd0c36bd22b74fbc01276957be