detect and warn when users try to use -r with a directory

[roycewilliams]

I discovered this issue while researching #1185.

Reading all rules in a directory was a recent feature request in #1182 . New users may try this. If they do, instead of rejecting the attempt early, hashcat goes through the full setup phase (hash loading, hardware initialization, potfile processing, etc.) and then pauses for a very long time at "Generated bitmap tables":

$./hashcat -m 100  -a 0 -r /var/tmp/emptydir/ target.hashes
hashcat (v3.40-49-g20057d8) starting...

[snip]

Generated bitmap tables...^C

It appears that hashcat may be trying to parse the directory as a ruleset? Not sure. If so, this could have weird side effects we should avoid.

Suggested solution: check if -r parameter is a directory, abort early, and suggest to the user the alternative that @jsteube suggested in #1182, but phrased to be platform-independent - someting like "Rules cannot be loaded from a directory. Concatenate your rules into a single file instead". This could head off a FAQ.