core: use variable "items_count" directly from hashtable without calling function hashtable_get_integer
core: in case of empty passphrase, keep encrypted data in memory until the user runs /secure decrypt (with the good passphrase)

If user gives no passphrase (for example one space when prompted), the encrypted
data read in sec.conf will be stored in a separate hashtable.
While this hashtable contains something, it is not allowed to do any operation
on passphrase or secured data (and it is not allowed to reload sec.conf).
The user can decrypt data with /secure decrypt <passphrase>.
rmodifier: update default rmodifier for /secure (add option decrypt)