nsjail for HOJ Beta 2

This is a version bump for nsjail for HOJ. This release follows nsjail v3.3.

• Basically rebase everything I have done in beta 1 to nsjail v3. Please refer to the previous release note.
• Port memory control group and memory statistic report to cgroup v2. The keys in LOG_STAT stays unchanged for backward-compatibility, despite that v2 has different terminologies. You may be interested at the documentation of cgroup v2.

nsjail for HOJ Beta 1

This release follows nsjail 2.7-42-g122f251 in upstream. Read full diff. (The version string is from git discribe)

• Attempt to raise real-time (SCHED_FIFO) priority to hard limit if possible (max for parent, max-1 for children)
• Call prctl(PR_SET_DUMPABLE, 1) to make the process dumpable by default (avoid failures when setuid but not root)
• Add some extra machine-readable messages meant for judging (still WIP), which are printed even in really quiet mode. For more details, visit this wiki page of the judge.
• Hardcode swappiness in memory control groups to zero to allow determining MLE by examining failcnt.
• Fix some inconsistencies in docs/configs/help.