linkbox: fix list paging and optimized synchronization.

1. The maximum number of objects on a page should be no more than
1000. Currently it is 1024, for this reason the listing always ends on
the first page with the error “object not found”, rclone tries to
upload the file again, Linkbox stores it with the name “filename(N)”,
and so the storage fills up indefinitely.

2. A hyphen is added to the list of allowed characters, that makes
queries more optimized (no need to load all files in a directory for
an entity with a hyphen).

backend/linkbox/linkbox.go

@@ -36,7 +36,7 @@ import (
 )
 
 const (
-	maxEntitiesPerPage = 1024
+	maxEntitiesPerPage = 1000
 	minSleep           = 200 * time.Millisecond
 	maxSleep           = 2 * time.Second
 	pacerBurst         = 1
@@ -220,7 +220,7 @@ type listAllFn func(*entity) bool
 //
 // If the name doesn't match this then do an dir list instead
 // N.B.: Linkbox doesn't support search by name that is longer than 50 chars
-var searchOK = regexp.MustCompile(`^[a-zA-Z0-9_ .]{1,50}$`)
+var searchOK = regexp.MustCompile(`^[a-zA-Z0-9_ -.]{1,50}$`)
 
 // Lists the directory required calling the user function on each item found
 //