* Remove `RIZIN_EXTRAS` because useless
Extras plugin should be treated as any other regular plugin, they should
not have their own directory. I believe we did not have anyway any
plugin using that directory.
* Do not use `<version>` in `/usr/share`
Users can only install one version of Rizin at a time anyway, thus
splitting the `/usr/share/rizin` directories does not really bring
multi-version install possible. At the same time, we stick to what's
common for other tools, which do not use version in `/usr/share`
* Move plugin directory to `/usr/lib64/rizin/plugins`
* Remove `RZ_HOME_BIN` env var because not useful anymore
* Introduce `rz_util/rz_path` to provide rizin paths
* Use `rz_path_system_sdb_types` whenever it makes sense
* Use some others `rz_path_*` functions instead of custom ones
* Use `rz_path` APIs for themes, fortunes and signatures
* Remove `rz_sys_prefix`
* Reduce the number of defines needed to handle home and system paths
* Reduce the number of `rz_path` functions
* Make the lib debug depending on log level and `RZ_LOG_DEBUG`
* Refactor a bit how directory of plugins are loaded
* Keep supporting old plugins directory for at least 0.4.0
