Avoid terminations when being `pid == 1`

[sisungo]

Currently, airupd will terminate if signals like SIGSEGV or SIGABRT are sent to the daemon, which usually causes a kernel panic. We should register signal handlers for them to avoid terminations. The signal handlers are simple enough, so they don't lead to UBs.

The signal handler should first try to execve:

airup debug --daemon-crash-handler

If the operation failed, try to execve on /bin/sh.

If all the above operations failed, do infinity spin loops.

This introduces a new airup debug utility, --daemon-crash-handler. This should also keep simple. The crash handler should:

• Set up new process reaper
• Print error message
• Do infinity loops to start shell

[hetaodo]

You're so good, sisungo

[sisungo]

Priority: Low.