This moves the fallible action of opening the file, from the
constructor, into the factory methods which can propagate any errors.
The wrinkle here is that failure to open a ConfigFile in read-only mode
is allowed (and expected, since the file may not exist), and treated as
if an empty file was successfully opened.
I've attempted to handle the errors gracefully where it was clear how to
do so, and simple, but a lot of this was just adding
`release_value_but_fixme_should_propagate_errors()` in places.
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.
SPDX License Identifiers are a more compact / standardized
way of representing file license information.
See: https://spdx.dev/resources/use/#identifiers
This was done with the `ambr` search and replace tool.
ambr --no-parent-ignore --key-from-file --rep-from-file key.txt rep.txt *
