Refactor init_system_thp_mode and print it in malloc stats.

[guangli-dai]

Two changes in this PR:

1. refactor so that init_system_thp_mode uses a different enum value. This is because thp_mode_t is designed for option opt.thp, which is not accurate: 0 should be "madvise" instead of "default". Besides, once HPA settles down, users should use HPA instead of PAC + opt.thp:always. This refactoring makes future removal easier and cleaner.
2. Export init_system_thp_mode value to stats. HPA interferes with kernel's THP settings and exporting it to stats help users understand the system better.

Updated malloc stats have a new section as below:

...
  config.xmalloc: false
System configuration
  system.thp_mode: "madvise"
Run-time option settings
...