chg: usr: Reduce memory footprint by actively returning unused memory to the OS

Previously, :iscman:`named` relied on the default allocator settings for
releasing unused memory back to the operating system, which could result in
unnecessarily high resident memory usage. :iscman:`named` now actively
manages memory page purging. On systems using jemalloc, background cleanup
threads are enabled and the dirty page decay time is reduced from 10 seconds
to 5 seconds. Additionally, a volume-based decay pass is triggered after
every 16 MiB of freed memory.  On glibc-based systems, a similar
volume-based mechanism using malloc_trim() is used instead.

Merge branch 'ondrej/enable-background-cleaning-of-unused-memory' into 'main'

See merge request isc-projects/bind9!11761