Make Xapian initialization easier to read

Define constants to make code more readable, specifically to make it more clear what _xapian_init_database() parameters mean.

If opening a writable database we call _xapian_init_database(XAPIAN_WRITABLE) instead of _xapian_init_database(TRUE). If flushing the database, we call _xapian_init_database(XAPIAN_WRITABLE, XAPIAN_FLUSH) instead of xapian_init_database(TRUE, TRUE). Purely cosmetic to make the code easier to follow.