exports: let kernel decide which flags vary by flavor

exports: let kernel decide which flags vary by flavor

Query the kernel to ask which flavors vary by pseudoflavor, and use that
instead of a fixed constant.  To allow the possibility of more flags
varying by pseudoflavor, use the set/clear_flags functions for all
options instead of setting some by hand.

Signed-off-by: J. Bruce Fields <bfields@citi.umich.edu>