New -noambiguous option to avoid look-alike characters

Hi, Chris!

Here's the tweak I emailed you about a while back to omit potentially misreadable characters from passwords -- this version uses a fixed set of lookalike characters, and copies the 'dist' and 'undist' hashes into new hashes with those characters omitted.

This change is intended for use when a printed list of passwords is being sent to a group of new users. The passwords need to conform to password complexity restrictions like String::MkPasswd provides, but we want to avoid calls to IT for help simply because a user can't tell an uppercase "I" from a lowercase "l."

I considered simply cloning the 'dist' and 'undist' hashes and manually removing the characters that are often confused (depending on font, depending on printer), but this is easier to test and revise. It does have a small CPU cost at startup, this way.

What do you think?

Jacob