Allow ';' as a Maildir flags seperator

Isync uses this by default on Windows where ':' is an invalid character
in file names. Also try to preserve the existing separator character
when generating a new file name.

man/mu-index.1

@@ -21,7 +21,7 @@ init\fR to initialize the database.
 \fBindex\fR understands Maildirs as defined by Daniel Bernstein for
 \fBqmail\fR(7). In addition, it understands recursive Maildirs (Maildirs
 within Maildirs), Maildir++. It can also deal with VFAT-based Maildirs
-which use '!' as the separators instead of ':'.
+which use '!' or ';' as the separators instead of ':'.
 
 E-mail messages which are not stored in something resembling a maildir
 leaf-directory (\fIcur\fR and \fInew\fR) are ignored, as are the cache