Fixes overflow bug in generate_password().

Can no longer create a possible infinite loop. Select next 
symbol based on the available pool, instead of filtering
given random results.

Also now allows any length of passwords. The password
length isn't limited by PASSWORD_SYMBOLS.

Fixes issue #386

git-svn-id: http://textpattern.googlecode.com/svn/development/4.x@5507 2fea6a4d-a838-0410-917b-93a53c48e9d2