WT_CONN_DATA_CORRUPTION incorrectly set in Btree verify code.

1. In bt_vrfy_dsk.c, WT_CONN_DATA_CORRUPTION is set in WT_RET_VRFY but not in WT_ERR_VRFY.

2. WT_CONN_DATA_CORRUPTION is set in __im_file_read(), which is the in-memory file system, so I don't see how corruption is possible in that case.

3. WT_CONN_DATA_CORRUPTION is set in __posix_file_read() and __win_file_read(); if we want to catch all the read functions, why not use __wt_read() instead of the underlying read OS-dependent read functions?