changes to MatSetOption() in petsc-dev

[Barry Smith]

  petsc-dev users,

    I have pushed a change to the MatSetOption() function.
It now takes an additional option PetscTruth flg, that either turns
on or turns off the option. The "turn-off" options are removed,
like MAT_NOT_SYMMETRIC etc. 

  Please run make all on PETSc after your next pull.

  Let petsc-maint at mcs.anl.gov know if you encounter any difficulties

   Barry