Problem:
========
- InnoDB fails to open undo tablespace when page0 is corrupted
and fails to throw error.

Solution:
=========
- InnoDB throws DB_CORRUPTION error when InnoDB encounters
page0 corruption of undo tablespace.

- InnoDB restores the page0 of undo tablespace from
doublewrite buffer if it encounters page corruption

- Moved Datafile::restore_from_doublewrite() to
recv_dblwr_t::restore_first_page(). So that undo
tablespace and system tablespace can use this function
instead of duplicating the code

srv_undo_tablespace_open(): Returns 0 if file doesn't exist
or ULINT_UNDEFINED if page0 is corrupted.