sending SIGHUP.

There were two problems:
  - after some recent fix, the server started to crash after
    receiving SIGHUP. That happened because LEX of new THD-object
    was not properly initialized.
  - user-specified log options were ignored when logs were reopened.

The fix is to 1) initialize LEX and 2) take user-specified options
into account.

There is no test case in this CS, because our test suite does not
support sending SIGHUP to the server.