D-Link switches improvement (inventorym, DDM, storage)

In this patch:

• includes/definitions/mibs/d-link.inc.php
  • AGENT-GENERAL-MIB - added discovery for storage from agentFLASHutilizationTable
  • ACLMGMT-MIB - unused (commented) - move comments for ACLMGMT-MIB after MIB definition
  • DDM-MGMT-MIB - deactivate definitions added in OBS-3256 (can't parse Threshold in definition, moved to includes/discovery/sensors/ddm-mgmt-mib.inc.php)
• includes/discovery/inventory/des3026-l2mgmt-mib.inc.php
  • DES3026-L2MGMT-MIB - inventory discovery with 2 slots
• includes/discovery/inventory/des3018-l2mgmt-mib.inc.php
  • DES3018-L2MGMT-MIB - symlink to des3026-l2mgmt-mib.inc.php
• includes/discovery/inventory/entity-mib.inc.php
  • improved inventory discovery by DGS3627G-L2MGMT-MIB and DGS3120-24SC-L2MGMT-MIB (like in OBS-3339)
• includes/discovery/sensors/ddm-mgmt-mib.inc.php
  • DDM-MGMT-MIB - add discovery for DDM sensors with Threshold (like OBS-3725 / OBS-3706 )
  • tested on D-Link
    • DGS-3627G 1A1G (firmware 3.03.B04)
    • DES-3200-28/C1 (firmware 4.46.B008)
    • DGS-3120-24SC B1 (firmware 3.12.R510)
    • DES-3526 A4 (firmware 6.20.B20/B21/B30)
    • DES-3550 A4G (firmware 6.20.B20)