opentracker.c: check set*id return values

This commit fix the following similar warnings:

    opentracker.c:562:7: warning: ignoring return value of ‘setegid’
    declared with attribute ‘warn_unused_result’ [-Wunused-result]
    […]

The man page of these functions ask users to explicitly check the return
value in case of any error happening.

opentracker.c

@@ -559,12 +559,20 @@ int drop_privileges ( const char * const serveruser, const char * const serverdi
     /* If we can't find server user, revert to nobody's default uid */
     if( !pws ) {
       fprintf( stderr, "Warning: Could not get password entry for %s. Reverting to uid -2.\n", serveruser );
-      setegid( (gid_t)-2 ); setgid( (gid_t)-2 );
-      setuid( (uid_t)-2 );  seteuid( (uid_t)-2 );
+      if (!setegid( (gid_t)-2 ) ||
+          !setgid( (gid_t)-2 ) ||
+          !setuid( (uid_t)-2 ) ||
+          !seteuid( (uid_t)-2 )) {
+        panic("Could not set uid to value -2");
+      }
     }
     else {
-      setegid( pws->pw_gid ); setgid( pws->pw_gid );
-      setuid( pws->pw_uid );  seteuid( pws->pw_uid );
+      if (!setegid( pws->pw_gid ) ||
+          !setgid( pws->pw_gid ) ||
+          !setuid( pws->pw_uid ) ||
+          !seteuid( pws->pw_uid )) {
+        panic("Could not set uid to specified value");
+      }
     }
 
     if( geteuid() == 0 || getegid() == 0 )