fs/Path: rename to AllocatedPath

The new class Path only holds a string pointer without being
responsible for allocation/deallocation.  The FileSystem.hxx library
accepts Path arguments instead of AllocatedPath, to avoid forcing
callers to allocate another string object.

Makefile.am

@@ -348,6 +348,7 @@ libfs_a_SOURCES = \
 	src/fs/Config.cxx src/fs/Config.hxx \
 	src/fs/Charset.cxx src/fs/Charset.hxx \
 	src/fs/Path.cxx src/fs/Path.hxx \
+	src/fs/AllocatedPath.cxx src/fs/AllocatedPath.hxx \
 	src/fs/FileSystem.cxx src/fs/FileSystem.hxx \
 	src/fs/DirectoryReader.hxx