[sandbox] Seccomp filter tweaks

* Restrict fcntl() to only the flags we need
* Non-fatally deny tgkill() and rt_sigaction() so that segfaults don't
  result in a SIGSYS. They're debuggable either way, but this way it's
  clearer what the issue is right away.
* Allow truncate() and ftruncate() for sqlite's alternate journal modes
* Slight macro cleanup
* Add missing colon in a DB log message

We don't need to worry about compilation problems arising if glibc or
musl-libc add their own wrapper for the seccomp() syscall in the future.
Ours will/would just silently take precedence over the external one
without interfering with compilation. This should work regardless of
whether libc uses weak symbols and regardless of whether libc is
dynamically or statically linked into the executable. The wrapper's
signature has been stripped of its static and inline qualifiers, as it
must match the exact declaration the libc headers will/would use.

Further, if a pre-compiled binary is run on a system which genuinely
doesn't support seccomp(), it'll just return ENOSYS and the server will
terminate with an error. The user can then just disable the sandbox in
the config file. We don't need any special logic for that scenario.

src/db/init.cpp

@@ -99,7 +99,7 @@ static void checkMetaTable() {
     sqlite3_stmt* stmt;
     sqlite3_prepare_v2(db, sql, -1, &stmt, NULL);
     if (sqlite3_step(stmt) != SQLITE_ROW) {
-        std::cout << "[FATAL] Failed to check meta table" << sqlite3_errmsg(db) << std::endl;
+        std::cout << "[FATAL] Failed to check meta table: " << sqlite3_errmsg(db) << std::endl;
         sqlite3_finalize(stmt);
         exit(1);
     }