tree-wide: lock in all memory pages when mlockall() is utilized, and on demand

tree-wide: lock in all memory pages when mlockall() is utilized, and on demand

When employing MCL_FUTURE we don't actually want it to impose
immediate population of malloc()-ed pages. Hence let's set
MCL_ONFAULT everywhere.

Additionally, specify MCL_CURRENT to ensure future memory allocations
on already mapped pages are covered too. (Addresses
https://github.com/systemd/systemd/pull/40319#discussion_r2693726196)

Note that in shutdown the mlockall() is done to avoid keeping swap space
busy, hence a dedicated call w/ MCL_CURRENT and w/o MCL_ONFAULT is made.