qsd: Add --daemonize

To implement this, we reuse the existing daemonizing functions from the
system emulator, which mainly do the following:
- Fork off a child process, and set up a pipe between parent and child
- The parent process waits until the child sends a status byte over the
  pipe (0 means that the child was set up successfully; anything else
  (including errors or EOF) means that the child was not set up
  successfully), and then exits with an appropriate exit status
- The child process enters a new session (forking off again), changes
  the umask, and will ignore terminal signals from then on
- Once set-up is complete, the child will chdir to /, redirect all
  standard I/O streams to /dev/null, and tell the parent that set-up has
  been completed successfully

In contrast to qemu-nbd's --fork implementation, during the set up
phase, error messages are not piped through the parent process.
qemu-nbd mainly does this to detect errors, though (while os_daemonize()
has the child explicitly signal success after set up); because we do not
redirect stderr after forking, error messages continue to appear on
whatever the parent's stderr was (until set up is complete).

Signed-off-by: Hanna Reitz <hreitz@redhat.com>
Message-Id: <20220303164814.284974-4-hreitz@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>

docs/tools/qemu-storage-daemon.rst

@@ -154,6 +154,13 @@ Standard options:
   created but before accepting connections. The daemon has started successfully
   when the pid file is written and clients may begin connecting.
 
+.. option:: --daemonize
+
+  Daemonize the process. The parent process will exit once startup is complete
+  (i.e., after the pid file has been or would have been written) or failure
+  occurs. Its exit code reflects whether the child has started up successfully
+  or failed to do so.
+
 Examples
 --------
 Launch the daemon with QMP monitor socket ``qmp.sock`` so clients can execute