Tracker: latent bugs surfaced by the new OMPI-layer unit tests

[jsquyres]

The new single-process OMPI-layer unit-test suite (#13979) surfaced several latent bugs in the OMPI/OPAL code base. This issue is the rollup tracker for those fixes; each has its own issue and its own PR against main.

• MPI_Type_get_contents crashes when the caller passes oversized output arrays #14051 — MPI_Type_get_contents crashes on oversized output arrays (PR fix(datatype): bound MPI_Type_get_contents loop by actual count #14055)
• Unaligned word-size accesses in packed datatype description (latent UB on strict-alignment arches) #14052 — unaligned word-size accesses in the packed datatype description (PR fix(datatype): always align packed-description word-size accesses #14056)
• MPI_Type_match_size can return composite or unavailable datatypes #14053 — MPI_Type_match_size can return composite/unavailable datatypes (PR fix(datatype): match only basic scalar types in ompi_datatype_match_size #14057)
• MPI-IO MPI_File_open fails on FreeBSD: sm sharedfp uses a non-portable POSIX semaphore name #14054 — MPI-IO MPI_File_open fails on FreeBSD: sm sharedfp semaphore name (PR fix(sharedfp/sm): use a POSIX-portable leading-slash semaphore name #14058)

The unit-test PR (#13979) is structured on top of these fixes and needs them to pass cleanly across macOS, Linux, and FreeBSD.