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/common.h

@@ -18,7 +18,7 @@
 
 #include "../boost-fix.h"
 
-#include <boost/filesystem.hpp>
+#include <ghc/filesystem.hpp>
 
 #include "../../common/logging/common.h"
 #include "../utils.h"
@@ -33,7 +33,7 @@
 class HostBridge {
    protected:
     HostBridge(MainContext& main_context,
-               boost::filesystem::path plugin_path,
+               ghc::filesystem::path plugin_path,
                pid_t parent_pid);
 
    public:
@@ -98,7 +98,7 @@ class HostBridge {
     /**
      * The path to the .dll being loaded in the Wine plugin host.
      */
-    const boost::filesystem::path plugin_path_;
+    const ghc::filesystem::path plugin_path_;
 
    protected:
     /**