executables: Reject Lua 5.1 early

Prevents attempting to load libraries that may no longer be found and
crashing with a traceback.

Platforms like Debian where multiple Lua versions can be installed at
the same time and 'lua' pointing to one of the installed interpreters
via symlinks, there's the possibility that prosody/prosodyctl may be
invoked with Lua 5.1, which will no longer have any of the rest of
Prosody libraries available to be require(), and thus would immediately
fail with an unfriendly traceback.

Checking and aborting early with a friendlier message and reference to
more information is better.

Part of #1600

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