core: introduce MemoryTHP= unit file setting

Transparent Hugepages (THP) is a Linux kernel feature that manages
memory using larger pages (2MB on x86, compared to the default 4KB).
The main goal is to improve memory management efficiency and system
performance, especially for memory-intensive applications.
However, it can cause drawbacks in some scenarios, such as memory
regression and latency spikes. THP policy is governed for the entire
system via /sys/kernel/mm/transparent_hugepage/enabled.
However, it can be overridden for individual workloads via prctl(2)
call.
MemoryTHP= is used to disable THPs at exec-invoke to stop
providing THPs for workloads where the drawbacks outweigh the advantages.
When set to "disable", MemoryTHP= disables THPs completely for the
process, irrespecitive of global THP controls.
When set to "madvise", MemoryTHP= disables THPs for the process except
when specifically madvised by the process with MADV_HUGEPAGE or MADV_COLLAPSE.

src/core/execute.h

@@ -311,6 +311,7 @@ typedef struct ExecContext {
         int mount_apivfs;
         int bind_log_sockets;
         int memory_ksm;
+        MemoryTHP memory_thp;
         PrivateTmp private_tmp;
         PrivateTmp private_var_tmp; /* This is not an independent parameter, but calculated from other
                                      * parameters in unit_patch_contexts(). */