Do PROLOG and fallback to CONTACT if supported

[eduardomozart]

Fix #851. I just flipped the "if" to do PROLOG first and fallback to CONTACT, this makes the Supported tasks be available on GLPI server before the inventory is made the first time.

[g-bougard]

Hi @eduardomozart

thank you for the attempt, but I will implement myself a fix, and be sure I still have some though on the right way to do it.

Actually, as this problem concerns the core part of glpi-agent and there are too many sensible parts around, I really prefer to handle it myself. Just be patient.

[eduardomozart]

No problem, just wanted to help. Not sure how to handle it properly if not this way because GLPI server seems to reject any first request attempt without proper PROLOG. I believe the solution you'll find will be similar to the one in this PR. Maybe create a variable to track last inventory date and if it doesn't exists do a PROLOG first, or as you said if it's a JSON do a PROLOG, but it could cause a PROLOG be sent twice with the code as it is today.