Fix MatConvert_MPIDense_Elemental() with non-trivial LDA

MatConvert_SeqAIJ_SeqDense: take into account leading dimension

MAT[SOLVERMUMPS|NEST]: add necessary MATDIAGONAL operations

MatRestoreRow(): avoid duplicated operations

Merge branch 'jacobf/2023-08-17/header-guard-check' into 'main'

Check header guards

See merge request petsc/petsc!6822

Convert all header guards to pragma once

Fix some malformed if !defined() header guards

Minor fixes

Add static to internal functions

Profiling: Improve !defined(PETSC_USE_LOG) #defines

- If we cast macro arguments to void, we can avoid unused errors that have
forced lots of conditionally included code like this:

```c
#if def

Profiling: Improve !defined(PETSC_USE_LOG) #defines

- If we cast macro arguments to void, we can avoid unused errors that have
forced lots of conditionally included code like this:

```c
#if defined(PETSC_USE_LOG)
PetscLogEvent event;
#endif
```

- Having global counters like petsc_TotalFlops externed event
if !PetscDefined(USE_LOG) helps to remove a lot of needless
`#ifdef`s around code that can instead be placed in
`if (PetscDefined(USE_LOG)) {}` blocks.

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Rename PetscValidPointer -> PetscAssertPointer

Deploy new PetscValidPointer, remove PetscValidXXXPointer

Lint apply: mat