Update brltty to version 5.1.
This version includes some interesting features. As an example, brltty now handles the screen in a different way, more performantly it seems.

I changed the package structure a little bit.
- added bash as as a dependency (inspired by brltty-minimal from AUR) :)
- dependencies are in alphabetical order now
- added doxygen as build-time dependency: so we get BRLAPI docs
- explicitly disable ocaml bindings building
Steps to reproduce:- disable lisp bindings building also like for ocaml
- included a dummy "revision_identifier.h" file: working around a small
problem in the build process; the call to the "getrevid" utility will fail if this isn't a git repository.
- synced brltty.service file from the upstream version, and introduced some changesallowing brltty to be gracefully stopped
- added shutdown.target to the Before= directive, to avoid also circular dependency, or at least this solves the problem (inspired from other DefaultDependency=no services)
Note that all changes in the service files are my own bugs propagated to upstream - going to communicate them.