FindMPI: Fix various legacy problems

This MR fixes a number of bugs stemming from legacy variable handling,
partially introduced by commit v3.10.0-rc1~103^2 (FindMPI: Add support
for FreeBSD and SLES, 2017-08-09).

* `MPI_COMPILE_OPTIONS` and `MPI_COMPILE_DEFINITIONS` legacy hints were not passed correctly.
* Legacy variables could be parsed after they were set by `FindMPI` cause erroneous hinting on subsequent runs.
* In the case of the compiler supporting MPI implicitly:
	* not all consumption variables would be correctly reset.
	* `MPI_<LANG>_HEADER_DIR` would be set unintentionally.
	* `MPI_<LANG>_LIB_NAMES` appeared in cache despite being ignored.
	* on subsequent passes, `MPI_<LANG>_COMPILER` could be corrupted.
* the C-to-CXX copy logic would take place even if `MPI_SKIP_GUESSING` was turned on
* if only some language compilers were given, inconsistent search behavior could be observed

Issue: #17538

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