Refactor db_helper_compare_days()

1. Function renamed to db_helper_compare_time()
2. It now accepts 4 parameters, which have been reordered
   • date 1
   • an SQL operator to use for the comparison
   • date 2
   • the number of seconds to compare against
     Note: the date parameters should only be strings (column names);
     date constants should be passed as DB parameters
3. The comparison is rewritten based on sign of $p_num_secs to avoid
   issues with unsigned integers on MySQL

Returns: date1 [operator] date2 + days

All occurences of the function in MantisBT code base have been updated
accordingly.

Fixes 0017980