genpasswd: New script, created by Satoshi Fukutomi

Downloaded from
<https://github.com/fuktommy/homebin/blob/master/genpasswd>. It uses the
same algorithm as the Android "genpasswd" utility, but the colon has to
be specified as part of the password if no URL is specified. Don't know
if that's a bug or feature. Need to test it more before merging it to
master.

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