Fix compiler warning (git_off_t cast to size_t).

Use size_t for page size, instead of long. Check result of sysconf.
Use size_t for page offset so no cast to size_t (second arg to p_mmap).
Use mod instead div/mult pair, so no cast to size_t is necessary.
Merge pull request #2366 from kitbellew/fix-indexer-mmap-cast

Fix compiler warning (git_off_t cast to size_t).