Replace most uses of Boost.{Filesystem,Process}

With the `ghc::filesystem` dependency from the previous commit. If we
can replace the rest of the Boost.Filesystem dependency then we can get
rid the one nasty runtime dependency we have, and it will make
implementing the chainloading simpler since can reuse more code without
bringing in Boost.

src/wine-host/bridges/group.h

@@ -123,7 +123,7 @@ class GroupBridge {
      *   STDOUT and STDERR streams of the current process will be redirected to
      *   a pipe so they can be properly written to a log file.
      */
-    explicit GroupBridge(boost::filesystem::path group_socket_path);
+    explicit GroupBridge(ghc::filesystem::path group_socket_path);
 
     ~GroupBridge() noexcept;