shutdownconfig: restrict chars allowed in password

http://www.murga-linux.com/puppy/viewtopic.php?p=789165#789165
> * Login Screen with the right keyboard language (someone encrypts his installation, then after reboot the password is not working! Bad start!)

This might be a real problem.
Well, apparently it happend to the above user, so not "might be" but "it is".

I can't imagine implementing a login screen with lang options in init right now, so I think the best workaround is to allow only common ASCII chars.

I'm sure everyone instantly noticed that there's lack of **`** (backtick) **"** (quotation mark), **\** (backslash) ** ** (space) and **'** (apostrophe).

First two because _pupdialog_ uses that darn **eval** and the above pair beautifully breaks the script, returning an empty string anyway.

Third, because when entering \ via 'read' (without '-r' param) at boot, it's interpreted as an escape character.

Similarily fourth (space), because if entered as first and/or last character, it won't be echo'ed at all.

And finally fifth (apostrophe), because I don't know why, but if it's there, we're getting kernel panic on next boot.

Phew...and I thought it will be "quick fix"...

Would be also nice to add a warning msg (after unsuccessful trial) with the list of allowed chars, but I'm unable to comprehend the code style "convention" used in this script.
Maybe later or perhaps someone else would try..?