afpd: Add "short name" (DOS 8.3 filename) support.

[NJRoadfan]

Note: This is a "stretch" goal.

This feature is solely used by old DOS based AFP clients such as AppleShare PC. Apple's AFP servers automatically derive a DOS compliant 8.3 filename to serve to these clients. Currently Netatalk has no support for this.

Challenges to implimentation include:

• Designing a robust shortname generation algorithm to avoid filename clashes.
• Maintaining the mapping between a host file and it's short filename. Currently the Netatalk EA structure doesn't support storing 8.3 filenames. We may be able to generate the short names on the fly with a FPEnumurate call and store them in the dircache structure. If a DOS client renames a file, it can overwrite the longer name with the 8.3 name. There are possible coherency issues if a file is changed by host FS activity or shared with Samba as well.