Fix termination of the daemon process during test, previously it was
the daemon instances were not cleaned up properly and remained running
after the tests
We used to collect all binaries into the `bin/` directory. However this
led to confusions since certain commands where available both as
`revng-command` and `revng command`.
This commit moves all the executables except `revng` into
`libexec/revng`, which, according to FHS, is dedicated to "internal
binaries that are not intended to be executed directly by users or shell
scripts".
Giacomo Vercesi