Manually close descriptors instead of using vfork

With `vfork()` the child process inherits the parents process image and
prevents copying them, but if it outlives its parent then the file
descriptors will still remain open. Manually closing all file
descriptors is the only solution here.

This was only an issue with Ardour since they don't open all of their
files with `FD_CLOEXEC`. Last update's watchdog timer somewhat mitigated
the issue, but Ardour should now no longer freeze when reopening because
of this. The watchdog timer is still necessary, since hanging Wine
processes will still prevent the Wine server from shutting down.

src/plugin/host-process.h

@@ -18,9 +18,6 @@
 
 #include <thread>
 
-// Boost.Process's auto detection for vfork() support doesn't seem to work
-#define BOOST_POSIX_HAS_VFORK 1
-
 #include <boost/asio/local/stream_protocol.hpp>
 #include <boost/asio/streambuf.hpp>
 #include <boost/filesystem.hpp>