crimson/alienstore: block SIGHUP to coexist with Seastar's signal handling.

crimson/alienstore: block SIGHUP to coexist with Seastar's signal handling.

In `crimson/osd/main.cc` we instruct Seastar to handle `SIGHUP`.

```
        // just ignore SIGHUP, we don't reread settings
        seastar::engine().handle_signal(SIGHUP, [] {})
```

This happens using the Seastar's signal handling infrastructure
which is incompliant with the alien world.

```
void
reactor::signals::handle_signal(int signo, noncopyable_function<void ()>&& handler) {
    // ...
    struct sigaction sa;
    sa.sa_sigaction = [](int sig, siginfo_t *info, void *p) {
        engine()._backend->signal_received(sig, info, p);
    };
    // ...
}
```

```
 extern __thread reactor* local_engine;
extern __thread size_t task_quota;

inline reactor& engine() {
    return *local_engine;
}
```

The low-level signal handler above assumes `local_engine._backend`
is not null which stays true only for threads from the S*'s world.
Unfortunately, as we don't block the `SIGHUP` for alien threads,
kernel is perfectly authorized to pick up one them to run the handler
leading to weirdly-looking segfaults like this one:

```
INFO  2021-04-23 07:06:57,807 [shard 0] bluestore - stat
DEBUG 2021-04-23 07:06:58,753 [shard 0] ms - [osd.1(client) v2:172.21.15.100:6802/30478@51064 >> mgr.4105 v2:172.21.15.109:6800/29891] --> #7 === pg_stats(0 pgs seq 55834574872 v 0) v2 (87)
...
INFO  2021-04-23 07:06:58,813 [shard 0] bluestore - stat
DEBUG 2021-04-23 07:06:59,753 [shard 0] osd - AdminSocket::handle_client: incoming asok string: {"prefix": "get_command_descriptions"}
INFO  2021-04-23 07:06:59,753 [shard 0] osd - asok response length: 2947
INFO  2021-04-23 07:06:59,817 [shard 0] bluestore - stat
DEBUG 2021-04-23 07:06:59,865 [shard 0] osd - AdminSocket::handle_client: incoming asok string: {"prefix": "get_command_descriptions"}
INFO  2021-04-23 07:06:59,866 [shard 0] osd - asok response length: 2947
DEBUG 2021-04-23 07:07:00,020 [shard 0] osd - AdminSocket::handle_client: incoming asok string: {"prefix": "get_command_descriptions"}
INFO  2021-04-23 07:07:00,020 [shard 0] osd - asok response length: 2947
INFO  2021-04-23 07:07:00,820 [shard 0] bluestore - stat
...
Backtrace:
 0# 0x00005600CD0D6AAF in ceph-osd
 1# FatalSignal::signaled(int) in ceph-osd
 2# FatalSignal::install_oneshot_signal_handler<11>()::{lambda(int, siginfo_t*, void*)#1}::_FUN(int, siginfo_t*, void*) in ceph-osd
 3# 0x00007F5877C7EB20 in /lib64/libpthread.so.0
 4# 0x00005600CD830B81 in ceph-osd
 5# 0x00007F5877C7EB20 in /lib64/libpthread.so.0
 6# pthread_cond_timedwait in /lib64/libpthread.so.0
 7# crimson::os::ThreadPool::loop(std::chrono::duration<long, std::ratio<1l, 1000l> >, unsigned long) in ceph-osd
 8# 0x00007F5877999BA3 in /lib64/libstdc++.so.6
 9# 0x00007F5877C7414A in /lib64/libpthread.so.0
10# clone in /lib64/libc.so.6
daemon-helper: command crashed with signal 11
```

Ultimately, it turned out the thread came out from a syscall (`futex`)
and started crunching the `SIGHUP` handler's code in which a nullptr
dereference happened.

This patch blocks `SIGHUP` for all threads spawned by `AlienStore`.

Signed-off-by: Radoslaw Zarzynski <rzarzyns@redhat.com>