Add server command to merge config by file name

[karoliineh]

Related to goblint/GobPie#32

Currently on Goblint conf file modification, GobPie kills the existing Goblint server and starts a new one:
...
This is quite undesirable since the incremental state of the server is lost. Instead GobPie should ask the Goblint server to just reload the conf file in the same process. A new Goblint server command needs to be added to merge a config by file name, instead of directly passing a JSON object to the server.

[sim642]

The config merging has a caveat that only becomes relevant in server mode. For example, consider the following:

1. Initially, by default, ana.int.interval is disabled.
2. The initial config from the --conf argument doesn't specify a value for ana.int.interval, so that's still disabled.
3. Then you change the config file to now enable ana.int.interval and load it, so it's now enabled.
4. But then you undo your change to the config file (remove ana.int.interval from it) and load it again. Even though you undid your change to the config, ana.int.interval still remains enabled!

This is because merging a config, that doesn't specify the option, doesn't change or reset it either (hence the merge). It could be surprising/confusing that the config you get isn't determined by the config files you have right now, but also the entire history of them.

Having another command for resetting options to their defaults is trivial, but that also means that server.enabled and server.reparse (or any other options initially specified on the command line) get disabled in the middle of server mode. So the reasonable consistent behavior is not that simple to achieve.