no : in filename for FAT filesystems & optionally allow-overwrite (for e.g. non-interactive use)

(This should be two pull requests, but I can't figure out this github interface.)

1. I'd prefer :-less filenames to make it possible to export directly to a cell phone

2. An optional argument allow-overwrite lets you put stuff like (bbdb-vcard-export "/network/file.vcf" 'all-records nil 'allow-overwrite) in a cron job