Revert "jemalloc: disable use of MADV_FREE"

This reverts commit fb47d3d4017a784c727b6748f55004426151de51.
Revert "Revert "The pmap function pmap_clear_reference() is no longer used.  Remove it.""

This reverts commit 042f76ff46d637bee75175cc42baa0e15cb820f8.
Revert "Revert "Significantly reduce the cost, i.e., run time, of calls to madvise(...,""

This reverts commit 49b506ffbfe73669278490e69287261719a69d96.
Revert "Partial revert "Merge projects/bhyve_npt_pmap into head.""

This reverts commit c3cb4c2ba0fdee362b45be3e2ad76c19ad6b3a8e.