index|extindex: support --dangerous flag

index|extindex: support --dangerous flag

This enables Xapian::DB_DANGEROUS to support in-place updates.
This can speed up the initial index and reduce I/O at the cost
of preventing concurrent readers and being unsafe in the face of
any abnormal terminations.  This is more dangerous than
--no-fsync.  --no-fsync is only unsafe in the event of a power
loss or kernel crash; --dangerous is unsafe even on SIGKILL.