This patch adds support for opening a ConfigFile using a file
descriptor rather than trying to open a the file by name directly.
In contrast to the previous implementation, ConfigFile now always keeps
a reference to an open File and does not reopen it for writing.
This requires providing an additional argument to open functions if a
file gets opened based on its name and the user of the api intends to
write to the file in the future.
2ea2d026c2