Add an optional function to the chsumvfs extension. When activated by

Add an optional function to the chsumvfs extension.  When activated by
the SQLITE_CKSUMVFS_INIT_FUNCNAME macro, this function will invoke the
file-control that sets the number of reserved bytes to 8.  This can be
used to initialize a cksumvfs database file by programming languages
that do not have access to the sqlite3_file_control() interface.

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