libcamera: process: Remove ProcessManager singleton

The `ProcessManager` is a singleton class to handle `SIGCHLD` signals
and report the exit status to the particular `Process` instance.

However, having a singleton in a library is not favourable and it is
even less favourable if it installs a signal handler.

Using pidfd it is possible to avoid the need for the signal handler;
and the `Process` objects can watch their pidfd themselves, eliminating
the need for the `ProcessManager` class altogether.

`P_PIDFD` for `waitid()` was introduced in Linux 5.4, so this change
raises the minimum supported kernel version. `clone3()`, `CLONE_PIDFD`,
`pidfd_send_signal()` were all introduced earlier.

Furthermore, the call to the `unshare()` system call can be removed
as those options can be passed to `clone3()` directly.

Signed-off-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

test/process/process_test.cpp

@@ -86,8 +86,6 @@ private:
 		exitCode_ = exitCode;
 	}
 
-	ProcessManager processManager_;
-
 	Process proc_;
 	enum Process::ExitStatus exitStatus_;
 	int exitCode_;