Some files had discordant executable bit presence and shebang presence.
This commit fixes these occurrences by adding removing the required
feautures where needed.
We introduce two paths where we can hard code search prefixes
(`/additional-search-prefixes`) and additional binaries paths
(`/share/revng/additional-bin-paths`).
This enables us to add to `additional-bin-paths` the configure-time
specified path of the preferred LLVM installation, instead of relying on
`PATH` to look up, e.g., `opt` at run-time.
When revng cli was invoked with a bare namespace, e.g. `revng ptml`
it would produce an ugly exception.
Fix this behavior by instead rerunning the command with `--help`
appended.
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