xsm, python tools: remove autogenerated xsm.py

xsm, python tools: remove autogenerated xsm.py

- The patch does away with the autogenerated xsm.py file and
introduces a config parameter in xend-config.sxp to determine the
security module. The parameter is (xsm_module_name {acm, dummy,
flask}).  The default setting/option is dummy.  .hgignore is also
updated to stop ignoring xsm.py on commits.

- The patch has created an xsconstant for XS_POLICY_FLASK and updated
  the toolchain to check the instance of XS_POLICY_USE.  XS_POLICY_USE
  evalauates to XS_POLICY_FLASK or XS_POLICY_ACM or XS_POLICY_DUMMY
  depending on configuration.

- Flask relies on the current value of ssidref returned by dominfo to
  ensure that the label to sid mapping is consistent.  ssidref had
  been pop'ed from the dominfo object.  The patch addresses this
  issue.

- Flask python module style cleanups.

Signed-off-by: George Coker <gscoker@alpha.ncsc.mil>