The default bahaviour for debug builds is to show dialog boxes for asserts
and invalid parameter handling. This is not so nice in general and causes
the Appveyor debug builds to hang because the io\file_seek.py test passes
a closed file descriptor to lseek. Disable this behaviour by printing
assert messages to the output instead of showing the dialog, and by
disabling 'invalid' parameter handling which causes the affected functions
to just return an error and set errno appropriately.