Some bugfixes for sensor/data code + add FCGI_INT_T

Fixed:
* Reject for invalid sensor id and/or format
* Get rid of dual FILE*; use single FILE* with mutex
* Out of bounds assertions on PrintByTimes/Indexes (esp. if user specifies large negative start_time/end_time)
* Mutex lock/unlock mismatch

Note: PrintByTimes/Indexes have exclusive ends now.
Todo:
* Check indeterminate behaviour of FindByTime (or just leave to client to deal with)
* Check if exclusive end is acceptable or not