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.

src/cryptsetup/cryptsetup.c

@@ -2582,7 +2582,7 @@ static int verb_attach(int argc, char *argv[], void *userdata) {
                   volume, source, strempty(arg_type), strempty(arg_cipher));
 
         /* A delicious drop of snake oil */
-        (void) mlockall(MCL_FUTURE);
+        (void) mlockall(MCL_CURRENT|MCL_FUTURE|MCL_ONFAULT);
 
         if (key_file && arg_keyfile_erase)
                 destroy_key_file = key_file; /* let's get this baby erased when we leave */