rrdtool: Keep time as 'long' despite 64bit time_t in musl

rrdtool: Keep time as 'long' despite 64bit time_t in musl

Adjust to the 64 bit time_t in musl 1.2.2 by keeping time
internally in RRD still as 'long', so that old databases
created during musl 1.1.x still work in 32 bit targets.

That also helps with several interal functions that use 'long'
for calculations and printf formatting.

* Define internal type TIME_INT as 'long', and use that
  internally in calculations
* Use time_t vs. TIME_INT casting when necessary
* Provide time_t variable pointers for functions expecting
  a time_t pointer.

Signed-off-by: Hannu Nyman <hannu.nyman@iki.fi>