Add para_upgrade_db.

Add para_upgrade_db.

This new executable is required to convert existing paraslash databases
to the new format. Only the layout of the audio file table changes
due to the switch from sha1 to sha256. The command creates and opens
a new audio file table and copies over each row, using an arbitrary
(incorrect) hash value for the values in hash column of the destination
table.

After all rows have been copied in this way, the old table is deleted
and the database directory is renamed from afs-database-0.4 to
afs-database-0.7, which will be the default path in paraslash-0.7.

Subsequent patches will modify para_server to load the database
from the new path and use sha256 instead of sha1 for the hash that
identifies the audio file. The user must then start the thusly patched
para_server and force-add all audio files to correct the hashes. This
approach keeps para_upgrade_db minimal and shortens its running time.