[Dovecot] Converting mbox files with dots in names to Maildir

[Kenneth Porter]

Looks like I need to use the optional convert-tool argument 
"alt_hierarchy_char=-" to get the desired effect. Looking at the source, 
this is used when the folder name is converted from source format to 
destination format and the name contains the hierarchy separator from the 
destination format.

Alas, if you don't provide this argument, convert-tool (and presumably the 
convert plugin, which shares this code) will just crash with a double-free 
when it encounters such a name.