[9.20] new: usr: add a rndc command to toggle jemalloc profiling

The new command is `rndc memprof`. The memory profiling status is also
reported inside `rndc status`. The status also shows whether named can
toggle memory profiling or not and if the server is built with jemalloc.

Closes #4759

Backport of MR !9370

Merge branch 'backport-4759-add-a-trigger-to-dump-jeprof-data-or-memory-statistics-9.20' into 'bind-9.20'

See merge request isc-projects/bind9!10000