Prune PETSc Exported Symbols - Sys
Introduce PETSC_SINGLE_LIBRARY_INTERN
that is PETSC_INTERN
if building single library, PETSC_EXTERN
otherwise and mark functions in petsc private headers with it if applicable. My metric with this is anything that does not have a sowing docstring is marked as min(PETSC_SINGLE_LIBRARY_INTERN,PETSC_INTERN)
. Functions with docstrings are internalized at discretion.
This MR does the PETSC_INTERN
pass over all "sys" headers.