Ensure strncpy'ed strings are all null terminated

If there is no null byte among the first n bytes of the source the
resulting string will not be properly null terminated.
Ensure that all strings that are copied via strncpy are properly
terminated copy "sizeof (dest) - 1" bytes and manually terminate
the string in the cases the array was not initialized.

Example compiler warning:
  ../daemon/gamemode-tests.c: In function ‘run_cpu_governor_tests’:
  ../daemon/gamemode-tests.c:326:4: warning: ‘strncpy’ specified bound
      256 equals destination size [-Wstringop-truncation]
    strncpy(defaultgov, currentgov, CONFIG_VALUE_MAX);
    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

daemon/governors-query.c

@@ -134,7 +134,7 @@ const char *get_gov_state(void)
 				return "malformed";
 			}
 
-			strncpy(governor, contents, sizeof(governor));
+			strncpy(governor, contents, sizeof(governor) - 1);
 		} else {
 			LOG_ERROR("Failed to read contents of %s\n", gov);
 		}